Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

- On Sep 19, 2017, at 5:47 PM, Andrea Parri parri.and...@gmail.com wrote:

> On Tue, Sep 19, 2017 at 03:56:31PM -0400, Mathieu Desnoyers wrote:
>> Document the membarrier requirement on having a full memory barrier in
>> __schedule() after coming from user-space, before storing to rq->curr.
>> It is provided by smp_mb__before_spinlock() in __schedule().
> 
> It is smp_mb__after_spinlock(). (Yes: I missed it in my previous email.)

OK. Will send a v3 fixing the changelog.

Thanks,

Mathieu

> 
>  Andrea
> 
> 
>> 
>> Document that membarrier requires a full barrier on transition from
>> kernel thread to userspace thread. We currently have an implicit barrier
>> from atomic_dec_and_test() in mmdrop() that ensures this.
>> 
>> The x86 switch_mm_irqs_off() full barrier is currently provided by many
>> cpumask update operations as well as write_cr3(). Document that
>> write_cr3() provides this barrier.
>> 
>> Changes since v1:
>> - Update comments to match reality for code paths which are after
>>   storing to rq->curr, before returning to user-space.
>> 
>> Signed-off-by: Mathieu Desnoyers 
>> CC: Peter Zijlstra 
>> CC: Paul E. McKenney 
>> CC: Boqun Feng 
>> CC: Andrew Hunter 
>> CC: Maged Michael 
>> CC: gro...@google.com
>> CC: Avi Kivity 
>> CC: Benjamin Herrenschmidt 
>> CC: Paul Mackerras 
>> CC: Michael Ellerman 
>> CC: Dave Watson 
>> ---
>>  arch/x86/mm/tlb.c| 5 +
>>  include/linux/sched/mm.h | 5 +
>>  kernel/sched/core.c  | 9 +
>>  3 files changed, 19 insertions(+)
>> 
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 1ab3821f9e26..74f94fe4aded 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct
>> mm_struct *next,
>>  }
>>  #endif
>>  
>> +/*
>> + * The membarrier system call requires a full memory barrier
>> + * before returning to user-space, after storing to rq->curr.
>> + * Writing to CR3 provides that full memory barrier.
>> + */
>>  if (real_prev == next) {
>>  VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
>>next->context.ctx_id);
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 3a19c253bdb1..766cc47c4d7c 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>>  extern void __mmdrop(struct mm_struct *);
>>  static inline void mmdrop(struct mm_struct *mm)
>>  {
>> +/*
>> + * The implicit full barrier implied by atomic_dec_and_test is
>> + * required by the membarrier system call before returning to
>> + * user-space, after storing to rq->curr.
>> + */
>>  if (unlikely(atomic_dec_and_test(>mm_count)))
>>  __mmdrop(mm);
>>  }
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 18a6966567da..7977b25acf54 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct 
>> task_struct
>> *prev)
>>  finish_arch_post_lock_switch();
>>  
>>  fire_sched_in_preempt_notifiers(current);
>> +/*
>> + * When transitioning from a kernel thread to a userspace
>> + * thread, mmdrop()'s implicit full barrier is required by the
>> + * membarrier system call, because the current active_mm can
>> + * become the current mm without going through switch_mm().
>> + */
>>  if (mm)
>>  mmdrop(mm);
>>  if (unlikely(prev_state == TASK_DEAD)) {
>> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
>>   * Make sure that signal_pending_state()->signal_pending() below
>>   * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>>   * done by the caller to avoid the race with signal_wake_up().
>> + *
>> + * The membarrier system call requires a full memory barrier
>> + * after coming from user-space, before storing to rq->curr.
>>   */
>>  rq_lock(rq, );
>>  smp_mb__after_spinlock();
>> --
>> 2.11.0

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

- On Sep 19, 2017, at 5:47 PM, Andrea Parri parri.and...@gmail.com wrote:

> On Tue, Sep 19, 2017 at 03:56:31PM -0400, Mathieu Desnoyers wrote:
>> Document the membarrier requirement on having a full memory barrier in
>> __schedule() after coming from user-space, before storing to rq->curr.
>> It is provided by smp_mb__before_spinlock() in __schedule().
> 
> It is smp_mb__after_spinlock(). (Yes: I missed it in my previous email.)

OK. Will send a v3 fixing the changelog.

Thanks,

Mathieu

> 
>  Andrea
> 
> 
>> 
>> Document that membarrier requires a full barrier on transition from
>> kernel thread to userspace thread. We currently have an implicit barrier
>> from atomic_dec_and_test() in mmdrop() that ensures this.
>> 
>> The x86 switch_mm_irqs_off() full barrier is currently provided by many
>> cpumask update operations as well as write_cr3(). Document that
>> write_cr3() provides this barrier.
>> 
>> Changes since v1:
>> - Update comments to match reality for code paths which are after
>>   storing to rq->curr, before returning to user-space.
>> 
>> Signed-off-by: Mathieu Desnoyers 
>> CC: Peter Zijlstra 
>> CC: Paul E. McKenney 
>> CC: Boqun Feng 
>> CC: Andrew Hunter 
>> CC: Maged Michael 
>> CC: gro...@google.com
>> CC: Avi Kivity 
>> CC: Benjamin Herrenschmidt 
>> CC: Paul Mackerras 
>> CC: Michael Ellerman 
>> CC: Dave Watson 
>> ---
>>  arch/x86/mm/tlb.c| 5 +
>>  include/linux/sched/mm.h | 5 +
>>  kernel/sched/core.c  | 9 +
>>  3 files changed, 19 insertions(+)
>> 
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 1ab3821f9e26..74f94fe4aded 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct
>> mm_struct *next,
>>  }
>>  #endif
>>  
>> +/*
>> + * The membarrier system call requires a full memory barrier
>> + * before returning to user-space, after storing to rq->curr.
>> + * Writing to CR3 provides that full memory barrier.
>> + */
>>  if (real_prev == next) {
>>  VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
>>next->context.ctx_id);
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 3a19c253bdb1..766cc47c4d7c 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>>  extern void __mmdrop(struct mm_struct *);
>>  static inline void mmdrop(struct mm_struct *mm)
>>  {
>> +/*
>> + * The implicit full barrier implied by atomic_dec_and_test is
>> + * required by the membarrier system call before returning to
>> + * user-space, after storing to rq->curr.
>> + */
>>  if (unlikely(atomic_dec_and_test(>mm_count)))
>>  __mmdrop(mm);
>>  }
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 18a6966567da..7977b25acf54 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct 
>> task_struct
>> *prev)
>>  finish_arch_post_lock_switch();
>>  
>>  fire_sched_in_preempt_notifiers(current);
>> +/*
>> + * When transitioning from a kernel thread to a userspace
>> + * thread, mmdrop()'s implicit full barrier is required by the
>> + * membarrier system call, because the current active_mm can
>> + * become the current mm without going through switch_mm().
>> + */
>>  if (mm)
>>  mmdrop(mm);
>>  if (unlikely(prev_state == TASK_DEAD)) {
>> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
>>   * Make sure that signal_pending_state()->signal_pending() below
>>   * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>>   * done by the caller to avoid the race with signal_wake_up().
>> + *
>> + * The membarrier system call requires a full memory barrier
>> + * after coming from user-space, before storing to rq->curr.
>>   */
>>  rq_lock(rq, );
>>  smp_mb__after_spinlock();
>> --
>> 2.11.0

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Andrea Parri]

On Tue, Sep 19, 2017 at 03:56:31PM -0400, Mathieu Desnoyers wrote:
> Document the membarrier requirement on having a full memory barrier in
> __schedule() after coming from user-space, before storing to rq->curr.
> It is provided by smp_mb__before_spinlock() in __schedule().

It is smp_mb__after_spinlock(). (Yes: I missed it in my previous email.)

  Andrea


> 
> Document that membarrier requires a full barrier on transition from
> kernel thread to userspace thread. We currently have an implicit barrier
> from atomic_dec_and_test() in mmdrop() that ensures this.
> 
> The x86 switch_mm_irqs_off() full barrier is currently provided by many
> cpumask update operations as well as write_cr3(). Document that
> write_cr3() provides this barrier.
> 
> Changes since v1:
> - Update comments to match reality for code paths which are after
>   storing to rq->curr, before returning to user-space.
> 
> Signed-off-by: Mathieu Desnoyers 
> CC: Peter Zijlstra 
> CC: Paul E. McKenney 
> CC: Boqun Feng 
> CC: Andrew Hunter 
> CC: Maged Michael 
> CC: gro...@google.com
> CC: Avi Kivity 
> CC: Benjamin Herrenschmidt 
> CC: Paul Mackerras 
> CC: Michael Ellerman 
> CC: Dave Watson 
> ---
>  arch/x86/mm/tlb.c| 5 +
>  include/linux/sched/mm.h | 5 +
>  kernel/sched/core.c  | 9 +
>  3 files changed, 19 insertions(+)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..74f94fe4aded 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
> mm_struct *next,
>   }
>  #endif
>  
> + /*
> +  * The membarrier system call requires a full memory barrier
> +  * before returning to user-space, after storing to rq->curr.
> +  * Writing to CR3 provides that full memory barrier.
> +  */
>   if (real_prev == next) {
>   VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> next->context.ctx_id);
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 3a19c253bdb1..766cc47c4d7c 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>  extern void __mmdrop(struct mm_struct *);
>  static inline void mmdrop(struct mm_struct *mm)
>  {
> + /*
> +  * The implicit full barrier implied by atomic_dec_and_test is
> +  * required by the membarrier system call before returning to
> +  * user-space, after storing to rq->curr.
> +  */
>   if (unlikely(atomic_dec_and_test(>mm_count)))
>   __mmdrop(mm);
>  }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 18a6966567da..7977b25acf54 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct 
> task_struct *prev)
>   finish_arch_post_lock_switch();
>  
>   fire_sched_in_preempt_notifiers(current);
> + /*
> +  * When transitioning from a kernel thread to a userspace
> +  * thread, mmdrop()'s implicit full barrier is required by the
> +  * membarrier system call, because the current active_mm can
> +  * become the current mm without going through switch_mm().
> +  */
>   if (mm)
>   mmdrop(mm);
>   if (unlikely(prev_state == TASK_DEAD)) {
> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   rq_lock(rq, );
>   smp_mb__after_spinlock();
> -- 
> 2.11.0
> 

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Andrea Parri]

On Tue, Sep 19, 2017 at 03:56:31PM -0400, Mathieu Desnoyers wrote:
> Document the membarrier requirement on having a full memory barrier in
> __schedule() after coming from user-space, before storing to rq->curr.
> It is provided by smp_mb__before_spinlock() in __schedule().

It is smp_mb__after_spinlock(). (Yes: I missed it in my previous email.)

  Andrea


> 
> Document that membarrier requires a full barrier on transition from
> kernel thread to userspace thread. We currently have an implicit barrier
> from atomic_dec_and_test() in mmdrop() that ensures this.
> 
> The x86 switch_mm_irqs_off() full barrier is currently provided by many
> cpumask update operations as well as write_cr3(). Document that
> write_cr3() provides this barrier.
> 
> Changes since v1:
> - Update comments to match reality for code paths which are after
>   storing to rq->curr, before returning to user-space.
> 
> Signed-off-by: Mathieu Desnoyers 
> CC: Peter Zijlstra 
> CC: Paul E. McKenney 
> CC: Boqun Feng 
> CC: Andrew Hunter 
> CC: Maged Michael 
> CC: gro...@google.com
> CC: Avi Kivity 
> CC: Benjamin Herrenschmidt 
> CC: Paul Mackerras 
> CC: Michael Ellerman 
> CC: Dave Watson 
> ---
>  arch/x86/mm/tlb.c| 5 +
>  include/linux/sched/mm.h | 5 +
>  kernel/sched/core.c  | 9 +
>  3 files changed, 19 insertions(+)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..74f94fe4aded 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
> mm_struct *next,
>   }
>  #endif
>  
> + /*
> +  * The membarrier system call requires a full memory barrier
> +  * before returning to user-space, after storing to rq->curr.
> +  * Writing to CR3 provides that full memory barrier.
> +  */
>   if (real_prev == next) {
>   VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> next->context.ctx_id);
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 3a19c253bdb1..766cc47c4d7c 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>  extern void __mmdrop(struct mm_struct *);
>  static inline void mmdrop(struct mm_struct *mm)
>  {
> + /*
> +  * The implicit full barrier implied by atomic_dec_and_test is
> +  * required by the membarrier system call before returning to
> +  * user-space, after storing to rq->curr.
> +  */
>   if (unlikely(atomic_dec_and_test(>mm_count)))
>   __mmdrop(mm);
>  }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 18a6966567da..7977b25acf54 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct 
> task_struct *prev)
>   finish_arch_post_lock_switch();
>  
>   fire_sched_in_preempt_notifiers(current);
> + /*
> +  * When transitioning from a kernel thread to a userspace
> +  * thread, mmdrop()'s implicit full barrier is required by the
> +  * membarrier system call, because the current active_mm can
> +  * become the current mm without going through switch_mm().
> +  */
>   if (mm)
>   mmdrop(mm);
>   if (unlikely(prev_state == TASK_DEAD)) {
> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   rq_lock(rq, );
>   smp_mb__after_spinlock();
> -- 
> 2.11.0
> 

[PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__before_spinlock() in __schedule().

Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread. We currently have an implicit barrier
from atomic_dec_and_test() in mmdrop() that ensures this.

The x86 switch_mm_irqs_off() full barrier is currently provided by many
cpumask update operations as well as write_cr3(). Document that
write_cr3() provides this barrier.

Changes since v1:
- Update comments to match reality for code paths which are after
  storing to rq->curr, before returning to user-space.

Signed-off-by: Mathieu Desnoyers 
CC: Peter Zijlstra 
CC: Paul E. McKenney 
CC: Boqun Feng 
CC: Andrew Hunter 
CC: Maged Michael 
CC: gro...@google.com
CC: Avi Kivity 
CC: Benjamin Herrenschmidt 
CC: Paul Mackerras 
CC: Michael Ellerman 
CC: Dave Watson 
---
 arch/x86/mm/tlb.c| 5 +
 include/linux/sched/mm.h | 5 +
 kernel/sched/core.c  | 9 +
 3 files changed, 19 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 1ab3821f9e26..74f94fe4aded 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
mm_struct *next,
}
 #endif
 
+   /*
+* The membarrier system call requires a full memory barrier
+* before returning to user-space, after storing to rq->curr.
+* Writing to CR3 provides that full memory barrier.
+*/
if (real_prev == next) {
VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
  next->context.ctx_id);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3a19c253bdb1..766cc47c4d7c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
 extern void __mmdrop(struct mm_struct *);
 static inline void mmdrop(struct mm_struct *mm)
 {
+   /*
+* The implicit full barrier implied by atomic_dec_and_test is
+* required by the membarrier system call before returning to
+* user-space, after storing to rq->curr.
+*/
if (unlikely(atomic_dec_and_test(>mm_count)))
__mmdrop(mm);
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 18a6966567da..7977b25acf54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct 
*prev)
finish_arch_post_lock_switch();
 
fire_sched_in_preempt_notifiers(current);
+   /*
+* When transitioning from a kernel thread to a userspace
+* thread, mmdrop()'s implicit full barrier is required by the
+* membarrier system call, because the current active_mm can
+* become the current mm without going through switch_mm().
+*/
if (mm)
mmdrop(mm);
if (unlikely(prev_state == TASK_DEAD)) {
@@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
 * Make sure that signal_pending_state()->signal_pending() below
 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 * done by the caller to avoid the race with signal_wake_up().
+*
+* The membarrier system call requires a full memory barrier
+* after coming from user-space, before storing to rq->curr.
 */
rq_lock(rq, );
smp_mb__after_spinlock();
-- 
2.11.0

[PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__before_spinlock() in __schedule().

Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread. We currently have an implicit barrier
from atomic_dec_and_test() in mmdrop() that ensures this.

The x86 switch_mm_irqs_off() full barrier is currently provided by many
cpumask update operations as well as write_cr3(). Document that
write_cr3() provides this barrier.

Changes since v1:
- Update comments to match reality for code paths which are after
  storing to rq->curr, before returning to user-space.

Signed-off-by: Mathieu Desnoyers 
CC: Peter Zijlstra 
CC: Paul E. McKenney 
CC: Boqun Feng 
CC: Andrew Hunter 
CC: Maged Michael 
CC: gro...@google.com
CC: Avi Kivity 
CC: Benjamin Herrenschmidt 
CC: Paul Mackerras 
CC: Michael Ellerman 
CC: Dave Watson 
---
 arch/x86/mm/tlb.c| 5 +
 include/linux/sched/mm.h | 5 +
 kernel/sched/core.c  | 9 +
 3 files changed, 19 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 1ab3821f9e26..74f94fe4aded 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
mm_struct *next,
}
 #endif
 
+   /*
+* The membarrier system call requires a full memory barrier
+* before returning to user-space, after storing to rq->curr.
+* Writing to CR3 provides that full memory barrier.
+*/
if (real_prev == next) {
VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
  next->context.ctx_id);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3a19c253bdb1..766cc47c4d7c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
 extern void __mmdrop(struct mm_struct *);
 static inline void mmdrop(struct mm_struct *mm)
 {
+   /*
+* The implicit full barrier implied by atomic_dec_and_test is
+* required by the membarrier system call before returning to
+* user-space, after storing to rq->curr.
+*/
if (unlikely(atomic_dec_and_test(>mm_count)))
__mmdrop(mm);
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 18a6966567da..7977b25acf54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct 
*prev)
finish_arch_post_lock_switch();
 
fire_sched_in_preempt_notifiers(current);
+   /*
+* When transitioning from a kernel thread to a userspace
+* thread, mmdrop()'s implicit full barrier is required by the
+* membarrier system call, because the current active_mm can
+* become the current mm without going through switch_mm().
+*/
if (mm)
mmdrop(mm);
if (unlikely(prev_state == TASK_DEAD)) {
@@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
 * Make sure that signal_pending_state()->signal_pending() below
 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 * done by the caller to avoid the race with signal_wake_up().
+*
+* The membarrier system call requires a full memory barrier
+* after coming from user-space, before storing to rq->curr.
 */
rq_lock(rq, );
smp_mb__after_spinlock();
-- 
2.11.0

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Paul E. McKenney]

On Mon, Aug 21, 2017 at 10:42:36AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> > @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
> >  * Make sure that signal_pending_state()->signal_pending() below
> >  * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> >  * done by the caller to avoid the race with signal_wake_up().
> > +*
> > +* The membarrier system call requires a full memory barrier
> > +* after coming from user-space, before storing to rq->curr.
> >  */
> > smp_mb__before_spinlock();
> > rq_lock(rq, );
> 
> Merge conflict here, current tip is over to smp_mb__after_spinlock().

Would some tree other than -rcu be better for that commit?

Thanx, Paul

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Paul E. McKenney]

On Mon, Aug 21, 2017 at 10:42:36AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> > @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
> >  * Make sure that signal_pending_state()->signal_pending() below
> >  * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> >  * done by the caller to avoid the race with signal_wake_up().
> > +*
> > +* The membarrier system call requires a full memory barrier
> > +* after coming from user-space, before storing to rq->curr.
> >  */
> > smp_mb__before_spinlock();
> > rq_lock(rq, );
> 
> Merge conflict here, current tip is over to smp_mb__after_spinlock().

Would some tree other than -rcu be better for that commit?

Thanx, Paul

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Peter Zijlstra]

On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   smp_mb__before_spinlock();
>   rq_lock(rq, );


Merge conflict here, current tip is over to smp_mb__after_spinlock().

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Peter Zijlstra]

On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   smp_mb__before_spinlock();
>   rq_lock(rq, );


Merge conflict here, current tip is over to smp_mb__after_spinlock().

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Paul E. McKenney]

On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> Document the membarrier requirement on having a full memory barrier in
> __schedule() after coming from user-space, before storing to rq->curr.
> It is provided by smp_mb__before_spinlock() in __schedule().
> 
> Document that membarrier requires a full barrier on transition from
> kernel thread to userspace thread, which skips the call to switch_mm(). We
> currently have an implicit barrier from atomic_dec_and_test() in mmdrop() that
> ensures this.
> 
> The x86 switch_mm_irqs_off() full barrier is currently provided by many 
> cpumask
> update operations as well as load_cr3(). Document that load_cr3() is providing
> this barrier.
> 
> [ Rebased on top of linux-rcu for-mingo branch.
>   Applies on top of "membarrier: Provide expedited private command". ]

I have queued this for review and testing, thank you!

Thanx, Paul

> Signed-off-by: Mathieu Desnoyers 
> CC: Peter Zijlstra 
> CC: Paul E. McKenney 
> CC: Boqun Feng 
> CC: Andrew Hunter 
> CC: Maged Michael 
> CC: gro...@google.com
> CC: Avi Kivity 
> CC: Benjamin Herrenschmidt 
> CC: Paul Mackerras 
> CC: Michael Ellerman 
> CC: Dave Watson 
> ---
>  arch/x86/mm/tlb.c| 3 +++
>  include/linux/sched/mm.h | 4 
>  kernel/sched/core.c  | 9 +
>  3 files changed, 16 insertions(+)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 014d07a..cd815b6 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
> mm_struct *next,
>* and neither LOCK nor MFENCE orders them.
>* Fortunately, load_cr3() is serializing and gives the
>* ordering guarantee we need.
> +  *
> +  * This full barrier is also required by the membarrier
> +  * system call.
>*/
>   load_cr3(next->pgd);
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 2b24a69..fe29d06 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,10 @@ static inline void mmgrab(struct mm_struct *mm)
>  extern void __mmdrop(struct mm_struct *);
>  static inline void mmdrop(struct mm_struct *mm)
>  {
> + /*
> +  * The implicit full barrier implied by atomic_dec_and_test is
> +  * required by the membarrier system call.
> +  */
>   if (unlikely(atomic_dec_and_test(>mm_count)))
>   __mmdrop(mm);
>  }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3f29c6a..b0f199f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2654,6 +2654,12 @@ static struct rq *finish_task_switch(struct 
> task_struct *prev)
>   finish_arch_post_lock_switch();
> 
>   fire_sched_in_preempt_notifiers(current);
> + /*
> +  * When transitioning from a kernel thread to a userspace
> +  * thread, mmdrop()'s implicit full barrier is required by the
> +  * membarrier system call, because the current active_mm can
> +  * become the current mm without going through switch_mm().
> +  */
>   if (mm)
>   mmdrop(mm);
>   if (unlikely(prev_state == TASK_DEAD)) {
> @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   smp_mb__before_spinlock();
>   rq_lock(rq, );
> -- 
> 1.9.1
> 

Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Paul E. McKenney]

On Fri, Aug 18, 2017 at 09:39:16PM -0700, Mathieu Desnoyers wrote:
> Document the membarrier requirement on having a full memory barrier in
> __schedule() after coming from user-space, before storing to rq->curr.
> It is provided by smp_mb__before_spinlock() in __schedule().
> 
> Document that membarrier requires a full barrier on transition from
> kernel thread to userspace thread, which skips the call to switch_mm(). We
> currently have an implicit barrier from atomic_dec_and_test() in mmdrop() that
> ensures this.
> 
> The x86 switch_mm_irqs_off() full barrier is currently provided by many 
> cpumask
> update operations as well as load_cr3(). Document that load_cr3() is providing
> this barrier.
> 
> [ Rebased on top of linux-rcu for-mingo branch.
>   Applies on top of "membarrier: Provide expedited private command". ]

I have queued this for review and testing, thank you!

Thanx, Paul

> Signed-off-by: Mathieu Desnoyers 
> CC: Peter Zijlstra 
> CC: Paul E. McKenney 
> CC: Boqun Feng 
> CC: Andrew Hunter 
> CC: Maged Michael 
> CC: gro...@google.com
> CC: Avi Kivity 
> CC: Benjamin Herrenschmidt 
> CC: Paul Mackerras 
> CC: Michael Ellerman 
> CC: Dave Watson 
> ---
>  arch/x86/mm/tlb.c| 3 +++
>  include/linux/sched/mm.h | 4 
>  kernel/sched/core.c  | 9 +
>  3 files changed, 16 insertions(+)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 014d07a..cd815b6 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
> mm_struct *next,
>* and neither LOCK nor MFENCE orders them.
>* Fortunately, load_cr3() is serializing and gives the
>* ordering guarantee we need.
> +  *
> +  * This full barrier is also required by the membarrier
> +  * system call.
>*/
>   load_cr3(next->pgd);
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 2b24a69..fe29d06 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,10 @@ static inline void mmgrab(struct mm_struct *mm)
>  extern void __mmdrop(struct mm_struct *);
>  static inline void mmdrop(struct mm_struct *mm)
>  {
> + /*
> +  * The implicit full barrier implied by atomic_dec_and_test is
> +  * required by the membarrier system call.
> +  */
>   if (unlikely(atomic_dec_and_test(>mm_count)))
>   __mmdrop(mm);
>  }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3f29c6a..b0f199f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2654,6 +2654,12 @@ static struct rq *finish_task_switch(struct 
> task_struct *prev)
>   finish_arch_post_lock_switch();
> 
>   fire_sched_in_preempt_notifiers(current);
> + /*
> +  * When transitioning from a kernel thread to a userspace
> +  * thread, mmdrop()'s implicit full barrier is required by the
> +  * membarrier system call, because the current active_mm can
> +  * become the current mm without going through switch_mm().
> +  */
>   if (mm)
>   mmdrop(mm);
>   if (unlikely(prev_state == TASK_DEAD)) {
> @@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
>* Make sure that signal_pending_state()->signal_pending() below
>* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>* done by the caller to avoid the race with signal_wake_up().
> +  *
> +  * The membarrier system call requires a full memory barrier
> +  * after coming from user-space, before storing to rq->curr.
>*/
>   smp_mb__before_spinlock();
>   rq_lock(rq, );
> -- 
> 1.9.1
> 

[PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__before_spinlock() in __schedule().

Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread, which skips the call to switch_mm(). We
currently have an implicit barrier from atomic_dec_and_test() in mmdrop() that
ensures this.

The x86 switch_mm_irqs_off() full barrier is currently provided by many cpumask
update operations as well as load_cr3(). Document that load_cr3() is providing
this barrier.

[ Rebased on top of linux-rcu for-mingo branch.
  Applies on top of "membarrier: Provide expedited private command". ]

Signed-off-by: Mathieu Desnoyers 
CC: Peter Zijlstra 
CC: Paul E. McKenney 
CC: Boqun Feng 
CC: Andrew Hunter 
CC: Maged Michael 
CC: gro...@google.com
CC: Avi Kivity 
CC: Benjamin Herrenschmidt 
CC: Paul Mackerras 
CC: Michael Ellerman 
CC: Dave Watson 
---
 arch/x86/mm/tlb.c| 3 +++
 include/linux/sched/mm.h | 4 
 kernel/sched/core.c  | 9 +
 3 files changed, 16 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 014d07a..cd815b6 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
mm_struct *next,
 * and neither LOCK nor MFENCE orders them.
 * Fortunately, load_cr3() is serializing and gives the
 * ordering guarantee we need.
+*
+* This full barrier is also required by the membarrier
+* system call.
 */
load_cr3(next->pgd);
 
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2b24a69..fe29d06 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,10 @@ static inline void mmgrab(struct mm_struct *mm)
 extern void __mmdrop(struct mm_struct *);
 static inline void mmdrop(struct mm_struct *mm)
 {
+   /*
+* The implicit full barrier implied by atomic_dec_and_test is
+* required by the membarrier system call.
+*/
if (unlikely(atomic_dec_and_test(>mm_count)))
__mmdrop(mm);
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3f29c6a..b0f199f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2654,6 +2654,12 @@ static struct rq *finish_task_switch(struct task_struct 
*prev)
finish_arch_post_lock_switch();
 
fire_sched_in_preempt_notifiers(current);
+   /*
+* When transitioning from a kernel thread to a userspace
+* thread, mmdrop()'s implicit full barrier is required by the
+* membarrier system call, because the current active_mm can
+* become the current mm without going through switch_mm().
+*/
if (mm)
mmdrop(mm);
if (unlikely(prev_state == TASK_DEAD)) {
@@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
 * Make sure that signal_pending_state()->signal_pending() below
 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 * done by the caller to avoid the race with signal_wake_up().
+*
+* The membarrier system call requires a full memory barrier
+* after coming from user-space, before storing to rq->curr.
 */
smp_mb__before_spinlock();
rq_lock(rq, );
-- 
1.9.1

[PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__before_spinlock() in __schedule().

Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread, which skips the call to switch_mm(). We
currently have an implicit barrier from atomic_dec_and_test() in mmdrop() that
ensures this.

The x86 switch_mm_irqs_off() full barrier is currently provided by many cpumask
update operations as well as load_cr3(). Document that load_cr3() is providing
this barrier.

[ Rebased on top of linux-rcu for-mingo branch.
  Applies on top of "membarrier: Provide expedited private command". ]

Signed-off-by: Mathieu Desnoyers 
CC: Peter Zijlstra 
CC: Paul E. McKenney 
CC: Boqun Feng 
CC: Andrew Hunter 
CC: Maged Michael 
CC: gro...@google.com
CC: Avi Kivity 
CC: Benjamin Herrenschmidt 
CC: Paul Mackerras 
CC: Michael Ellerman 
CC: Dave Watson 
---
 arch/x86/mm/tlb.c| 3 +++
 include/linux/sched/mm.h | 4 
 kernel/sched/core.c  | 9 +
 3 files changed, 16 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 014d07a..cd815b6 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct 
mm_struct *next,
 * and neither LOCK nor MFENCE orders them.
 * Fortunately, load_cr3() is serializing and gives the
 * ordering guarantee we need.
+*
+* This full barrier is also required by the membarrier
+* system call.
 */
load_cr3(next->pgd);
 
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2b24a69..fe29d06 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,10 @@ static inline void mmgrab(struct mm_struct *mm)
 extern void __mmdrop(struct mm_struct *);
 static inline void mmdrop(struct mm_struct *mm)
 {
+   /*
+* The implicit full barrier implied by atomic_dec_and_test is
+* required by the membarrier system call.
+*/
if (unlikely(atomic_dec_and_test(>mm_count)))
__mmdrop(mm);
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3f29c6a..b0f199f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2654,6 +2654,12 @@ static struct rq *finish_task_switch(struct task_struct 
*prev)
finish_arch_post_lock_switch();
 
fire_sched_in_preempt_notifiers(current);
+   /*
+* When transitioning from a kernel thread to a userspace
+* thread, mmdrop()'s implicit full barrier is required by the
+* membarrier system call, because the current active_mm can
+* become the current mm without going through switch_mm().
+*/
if (mm)
mmdrop(mm);
if (unlikely(prev_state == TASK_DEAD)) {
@@ -3295,6 +3301,9 @@ static void __sched notrace __schedule(bool preempt)
 * Make sure that signal_pending_state()->signal_pending() below
 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 * done by the caller to avoid the race with signal_wake_up().
+*
+* The membarrier system call requires a full memory barrier
+* after coming from user-space, before storing to rq->curr.
 */
smp_mb__before_spinlock();
rq_lock(rq, );
-- 
1.9.1