Re: [PATCH 1/4] x86, fpu: introduce per-cpu "bool in_kernel_fpu"

[Suresh Siddha]

On Fri, Aug 29, 2014 at 11:16 AM, Oleg Nesterov  wrote:
> interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
> is safe or not. In particulat it should obviously deny the nested
> kernel_fpu_begin() and this logic doesn't look clean.
>
> If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
> interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
> __thread_clear_has_fpu().
>
> Otherwise we demand that the interrupted task has no FPU if it is in
> kernel mode, this works becase __kernel_fpu_begin() does clts().
>
> Add the per-cpu "bool in_kernel_fpu" variable, and change this code
> to check/set/clear it. This allows to do some cleanups (see the next
> changes) and fixes.
>
> Note that the current code looks racy. Say, kernel_fpu_begin() right
> after math_state_restore()->__thread_fpu_begin() will overwrite the
> regs we are going to restore. This patch doesn't even try to fix this,

yes indeed, explicit calls to math_state_restore() in eager_fpu case
has this race. I guess this is present from the commit 5187b28f.

thanks,
suresh


> it just adds the comment, but "in_kernel_fpu" can also be used to
> implement kernel_fpu_disable() / kernel_fpu_enable().
>
> Signed-off-by: Oleg Nesterov 
> ---
>  arch/x86/include/asm/i387.h |2 +-
>  arch/x86/kernel/i387.c  |   10 ++
>  2 files changed, 11 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
> index ed8089d..5e275d3 100644
> --- a/arch/x86/include/asm/i387.h
> +++ b/arch/x86/include/asm/i387.h
> @@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);
>
>  static inline void kernel_fpu_begin(void)
>  {
> -   WARN_ON_ONCE(!irq_fpu_usable());
> preempt_disable();
> +   WARN_ON_ONCE(!irq_fpu_usable());
> __kernel_fpu_begin();
>  }
>
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
> index d5dd808..8fb8868 100644
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -19,6 +19,8 @@
>  #include 
>  #include 
>
> +static DEFINE_PER_CPU(bool, in_kernel_fpu);
> +
>  /*
>   * Were we in an interrupt that interrupted kernel mode?
>   *
> @@ -33,6 +35,9 @@
>   */
>  static inline bool interrupted_kernel_fpu_idle(void)
>  {
> +   if (this_cpu_read(in_kernel_fpu))
> +   return false;
> +
> if (use_eager_fpu())
> return __thread_has_fpu(current);
>
> @@ -73,6 +78,9 @@ void __kernel_fpu_begin(void)
>  {
> struct task_struct *me = current;
>
> +   this_cpu_write(in_kernel_fpu, true);
> +
> +   /* FIXME: race with math_state_restore()-like code */
> if (__thread_has_fpu(me)) {
> __thread_clear_has_fpu(me);
> __save_init_fpu(me);
> @@ -99,6 +107,8 @@ void __kernel_fpu_end(void)
> } else {
> stts();
> }
> +
> +   this_cpu_write(in_kernel_fpu, false);
>  }
>  EXPORT_SYMBOL(__kernel_fpu_end);
>
> --
> 1.5.5.1
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH 1/4] x86, fpu: introduce per-cpu bool in_kernel_fpu

[Suresh Siddha]

On Fri, Aug 29, 2014 at 11:16 AM, Oleg Nesterov o...@redhat.com wrote:
 interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
 is safe or not. In particulat it should obviously deny the nested
 kernel_fpu_begin() and this logic doesn't look clean.

 If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
 interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
 __thread_clear_has_fpu().

 Otherwise we demand that the interrupted task has no FPU if it is in
 kernel mode, this works becase __kernel_fpu_begin() does clts().

 Add the per-cpu bool in_kernel_fpu variable, and change this code
 to check/set/clear it. This allows to do some cleanups (see the next
 changes) and fixes.

 Note that the current code looks racy. Say, kernel_fpu_begin() right
 after math_state_restore()-__thread_fpu_begin() will overwrite the
 regs we are going to restore. This patch doesn't even try to fix this,

yes indeed, explicit calls to math_state_restore() in eager_fpu case
has this race. I guess this is present from the commit 5187b28f.

thanks,
suresh


 it just adds the comment, but in_kernel_fpu can also be used to
 implement kernel_fpu_disable() / kernel_fpu_enable().

 Signed-off-by: Oleg Nesterov o...@redhat.com
 ---
  arch/x86/include/asm/i387.h |2 +-
  arch/x86/kernel/i387.c  |   10 ++
  2 files changed, 11 insertions(+), 1 deletions(-)

 diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
 index ed8089d..5e275d3 100644
 --- a/arch/x86/include/asm/i387.h
 +++ b/arch/x86/include/asm/i387.h
 @@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);

  static inline void kernel_fpu_begin(void)
  {
 -   WARN_ON_ONCE(!irq_fpu_usable());
 preempt_disable();
 +   WARN_ON_ONCE(!irq_fpu_usable());
 __kernel_fpu_begin();
  }

 diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
 index d5dd808..8fb8868 100644
 --- a/arch/x86/kernel/i387.c
 +++ b/arch/x86/kernel/i387.c
 @@ -19,6 +19,8 @@
  #include asm/fpu-internal.h
  #include asm/user.h

 +static DEFINE_PER_CPU(bool, in_kernel_fpu);
 +
  /*
   * Were we in an interrupt that interrupted kernel mode?
   *
 @@ -33,6 +35,9 @@
   */
  static inline bool interrupted_kernel_fpu_idle(void)
  {
 +   if (this_cpu_read(in_kernel_fpu))
 +   return false;
 +
 if (use_eager_fpu())
 return __thread_has_fpu(current);

 @@ -73,6 +78,9 @@ void __kernel_fpu_begin(void)
  {
 struct task_struct *me = current;

 +   this_cpu_write(in_kernel_fpu, true);
 +
 +   /* FIXME: race with math_state_restore()-like code */
 if (__thread_has_fpu(me)) {
 __thread_clear_has_fpu(me);
 __save_init_fpu(me);
 @@ -99,6 +107,8 @@ void __kernel_fpu_end(void)
 } else {
 stts();
 }
 +
 +   this_cpu_write(in_kernel_fpu, false);
  }
  EXPORT_SYMBOL(__kernel_fpu_end);

 --
 1.5.5.1


--
To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[PATCH 1/4] x86, fpu: introduce per-cpu "bool in_kernel_fpu"

[Oleg Nesterov]

interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
is safe or not. In particulat it should obviously deny the nested
kernel_fpu_begin() and this logic doesn't look clean.

If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
__thread_clear_has_fpu().

Otherwise we demand that the interrupted task has no FPU if it is in
kernel mode, this works becase __kernel_fpu_begin() does clts().

Add the per-cpu "bool in_kernel_fpu" variable, and change this code
to check/set/clear it. This allows to do some cleanups (see the next
changes) and fixes.

Note that the current code looks racy. Say, kernel_fpu_begin() right
after math_state_restore()->__thread_fpu_begin() will overwrite the
regs we are going to restore. This patch doesn't even try to fix this,
it just adds the comment, but "in_kernel_fpu" can also be used to
implement kernel_fpu_disable() / kernel_fpu_enable().

Signed-off-by: Oleg Nesterov 
---
 arch/x86/include/asm/i387.h |2 +-
 arch/x86/kernel/i387.c  |   10 ++
 2 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index ed8089d..5e275d3 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);
 
 static inline void kernel_fpu_begin(void)
 {
-   WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
+   WARN_ON_ONCE(!irq_fpu_usable());
__kernel_fpu_begin();
 }
 
diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index d5dd808..8fb8868 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -19,6 +19,8 @@
 #include 
 #include 
 
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
@@ -33,6 +35,9 @@
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
+   if (this_cpu_read(in_kernel_fpu))
+   return false;
+
if (use_eager_fpu())
return __thread_has_fpu(current);
 
@@ -73,6 +78,9 @@ void __kernel_fpu_begin(void)
 {
struct task_struct *me = current;
 
+   this_cpu_write(in_kernel_fpu, true);
+
+   /* FIXME: race with math_state_restore()-like code */
if (__thread_has_fpu(me)) {
__thread_clear_has_fpu(me);
__save_init_fpu(me);
@@ -99,6 +107,8 @@ void __kernel_fpu_end(void)
} else {
stts();
}
+
+   this_cpu_write(in_kernel_fpu, false);
 }
 EXPORT_SYMBOL(__kernel_fpu_end);
 
-- 
1.5.5.1


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[PATCH 1/4] x86, fpu: introduce per-cpu bool in_kernel_fpu

[Oleg Nesterov]

interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
is safe or not. In particulat it should obviously deny the nested
kernel_fpu_begin() and this logic doesn't look clean.

If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
__thread_clear_has_fpu().

Otherwise we demand that the interrupted task has no FPU if it is in
kernel mode, this works becase __kernel_fpu_begin() does clts().

Add the per-cpu bool in_kernel_fpu variable, and change this code
to check/set/clear it. This allows to do some cleanups (see the next
changes) and fixes.

Note that the current code looks racy. Say, kernel_fpu_begin() right
after math_state_restore()-__thread_fpu_begin() will overwrite the
regs we are going to restore. This patch doesn't even try to fix this,
it just adds the comment, but in_kernel_fpu can also be used to
implement kernel_fpu_disable() / kernel_fpu_enable().

Signed-off-by: Oleg Nesterov o...@redhat.com
---
 arch/x86/include/asm/i387.h |2 +-
 arch/x86/kernel/i387.c  |   10 ++
 2 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index ed8089d..5e275d3 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);
 
 static inline void kernel_fpu_begin(void)
 {
-   WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
+   WARN_ON_ONCE(!irq_fpu_usable());
__kernel_fpu_begin();
 }
 
diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index d5dd808..8fb8868 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -19,6 +19,8 @@
 #include asm/fpu-internal.h
 #include asm/user.h
 
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
@@ -33,6 +35,9 @@
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
+   if (this_cpu_read(in_kernel_fpu))
+   return false;
+
if (use_eager_fpu())
return __thread_has_fpu(current);
 
@@ -73,6 +78,9 @@ void __kernel_fpu_begin(void)
 {
struct task_struct *me = current;
 
+   this_cpu_write(in_kernel_fpu, true);
+
+   /* FIXME: race with math_state_restore()-like code */
if (__thread_has_fpu(me)) {
__thread_clear_has_fpu(me);
__save_init_fpu(me);
@@ -99,6 +107,8 @@ void __kernel_fpu_end(void)
} else {
stts();
}
+
+   this_cpu_write(in_kernel_fpu, false);
 }
 EXPORT_SYMBOL(__kernel_fpu_end);
 
-- 
1.5.5.1


--
To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/