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 <mathieu.desnoy...@efficios.com>
> CC: Peter Zijlstra <pet...@infradead.org>
> CC: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.f...@gmail.com>
> CC: Andrew Hunter <a...@google.com>
> CC: Maged Michael <maged.mich...@gmail.com>
> CC: gro...@google.com
> CC: Avi Kivity <a...@scylladb.com>
> CC: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> CC: Paul Mackerras <pau...@samba.org>
> CC: Michael Ellerman <m...@ellerman.id.au>
> CC: Dave Watson <davejwat...@fb.com>
> ---
> 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->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, &rf);
> --
> 1.9.1
>
