Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Peter Zijlstra]

On Tue, Oct 04, 2016 at 08:57:55PM -0700, Davidlohr Bueso wrote:
> On Mon, 03 Oct 2016, Peter Zijlstra wrote:
> 
> >Since the futex_q can dissapear the instruction after assigning NULL,
> >this really should be a RELEASE barrier. That stops loads from hitting
> >dead memory too.
> >
> >Signed-off-by: Peter Zijlstra (Intel) 
> >---
> >kernel/futex.c |3 +--
> >1 file changed, 1 insertion(+), 2 deletions(-)
> >
> >--- a/kernel/futex.c
> >+++ b/kernel/futex.c
> >@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
> >  * memory barrier is required here to prevent the following
> >  * store to lock_ptr from getting ahead of the plist_del.
> >  */
> >-smp_wmb();
> >-q->lock_ptr = NULL;
> >+smp_store_release(>lock_ptr, NULL);
> >}
> 
> Hmm, what if we relied on the implicit barrier in the wake_q_add()
> above and got rid of the smp_wmb altogether? We'd obviously have to
> move up __unqueue_futex(), but all we care about is the publishing
> store to lock_ptr being the last operation, or at least the plist_del,
> such that the wakeup order is respected; ie:
> 
>   __unqueue_futex(q);
>   wake_q_add(wake_q, p);
>   q->lock_ptr = NULL;

Less obvious but would work I suppose, I didn't look too hard at the
ordering requirements on __unqueue_futex(), but an early morning glance
(without tea) doesn't show any obvious problems with that.

Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Peter Zijlstra]

On Tue, Oct 04, 2016 at 08:57:55PM -0700, Davidlohr Bueso wrote:
> On Mon, 03 Oct 2016, Peter Zijlstra wrote:
> 
> >Since the futex_q can dissapear the instruction after assigning NULL,
> >this really should be a RELEASE barrier. That stops loads from hitting
> >dead memory too.
> >
> >Signed-off-by: Peter Zijlstra (Intel) 
> >---
> >kernel/futex.c |3 +--
> >1 file changed, 1 insertion(+), 2 deletions(-)
> >
> >--- a/kernel/futex.c
> >+++ b/kernel/futex.c
> >@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
> >  * memory barrier is required here to prevent the following
> >  * store to lock_ptr from getting ahead of the plist_del.
> >  */
> >-smp_wmb();
> >-q->lock_ptr = NULL;
> >+smp_store_release(>lock_ptr, NULL);
> >}
> 
> Hmm, what if we relied on the implicit barrier in the wake_q_add()
> above and got rid of the smp_wmb altogether? We'd obviously have to
> move up __unqueue_futex(), but all we care about is the publishing
> store to lock_ptr being the last operation, or at least the plist_del,
> such that the wakeup order is respected; ie:
> 
>   __unqueue_futex(q);
>   wake_q_add(wake_q, p);
>   q->lock_ptr = NULL;

Less obvious but would work I suppose, I didn't look too hard at the
ordering requirements on __unqueue_futex(), but an early morning glance
(without tea) doesn't show any obvious problems with that.

Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Davidlohr Bueso]

On Mon, 03 Oct 2016, Peter Zijlstra wrote:


Since the futex_q can dissapear the instruction after assigning NULL,
this really should be a RELEASE barrier. That stops loads from hitting
dead memory too.

Signed-off-by: Peter Zijlstra (Intel) 
---
kernel/futex.c |3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
 * memory barrier is required here to prevent the following
 * store to lock_ptr from getting ahead of the plist_del.
 */
-   smp_wmb();
-   q->lock_ptr = NULL;
+   smp_store_release(>lock_ptr, NULL);
}


Hmm, what if we relied on the implicit barrier in the wake_q_add()
above and got rid of the smp_wmb altogether? We'd obviously have to
move up __unqueue_futex(), but all we care about is the publishing
store to lock_ptr being the last operation, or at least the plist_del,
such that the wakeup order is respected; ie:

  __unqueue_futex(q);
  wake_q_add(wake_q, p);
  q->lock_ptr = NULL;

Thanks,
Davidlohr

Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Davidlohr Bueso]

On Mon, 03 Oct 2016, Peter Zijlstra wrote:


Since the futex_q can dissapear the instruction after assigning NULL,
this really should be a RELEASE barrier. That stops loads from hitting
dead memory too.

Signed-off-by: Peter Zijlstra (Intel) 
---
kernel/futex.c |3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
 * memory barrier is required here to prevent the following
 * store to lock_ptr from getting ahead of the plist_del.
 */
-   smp_wmb();
-   q->lock_ptr = NULL;
+   smp_store_release(>lock_ptr, NULL);
}


Hmm, what if we relied on the implicit barrier in the wake_q_add()
above and got rid of the smp_wmb altogether? We'd obviously have to
move up __unqueue_futex(), but all we care about is the publishing
store to lock_ptr being the last operation, or at least the plist_del,
such that the wakeup order is respected; ie:

  __unqueue_futex(q);
  wake_q_add(wake_q, p);
  q->lock_ptr = NULL;

Thanks,
Davidlohr

Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Steven Rostedt]

On Mon, 03 Oct 2016 11:12:36 +0200
Peter Zijlstra  wrote:

> Since the futex_q can dissapear the instruction after assigning NULL,
> this really should be a RELEASE barrier. That stops loads from hitting
> dead memory too.
> 
> Signed-off-by: Peter Zijlstra (Intel) 
> ---
>  kernel/futex.c |3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
>* memory barrier is required here to prevent the following
>* store to lock_ptr from getting ahead of the plist_del.
>*/
> - smp_wmb();
> - q->lock_ptr = NULL;
> + smp_store_release(>lock_ptr, NULL);
>  }
>  
>  static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q 
> *top_waiter,
> 

Reviewed-by: Steven Rostedt 

-- Steve

Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Steven Rostedt]

On Mon, 03 Oct 2016 11:12:36 +0200
Peter Zijlstra  wrote:

> Since the futex_q can dissapear the instruction after assigning NULL,
> this really should be a RELEASE barrier. That stops loads from hitting
> dead memory too.
> 
> Signed-off-by: Peter Zijlstra (Intel) 
> ---
>  kernel/futex.c |3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
>* memory barrier is required here to prevent the following
>* store to lock_ptr from getting ahead of the plist_del.
>*/
> - smp_wmb();
> - q->lock_ptr = NULL;
> + smp_store_release(>lock_ptr, NULL);
>  }
>  
>  static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q 
> *top_waiter,
> 

Reviewed-by: Steven Rostedt 

-- Steve

[RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Peter Zijlstra]

Since the futex_q can dissapear the instruction after assigning NULL,
this really should be a RELEASE barrier. That stops loads from hitting
dead memory too.

Signed-off-by: Peter Zijlstra (Intel) 
---
 kernel/futex.c |3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
 * memory barrier is required here to prevent the following
 * store to lock_ptr from getting ahead of the plist_del.
 */
-   smp_wmb();
-   q->lock_ptr = NULL;
+   smp_store_release(>lock_ptr, NULL);
 }
 
 static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q 
*top_waiter,

[RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex()

[Peter Zijlstra]

Since the futex_q can dissapear the instruction after assigning NULL,
this really should be a RELEASE barrier. That stops loads from hitting
dead memory too.

Signed-off-by: Peter Zijlstra (Intel) 
---
 kernel/futex.c |3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_
 * memory barrier is required here to prevent the following
 * store to lock_ptr from getting ahead of the plist_del.
 */
-   smp_wmb();
-   q->lock_ptr = NULL;
+   smp_store_release(>lock_ptr, NULL);
 }
 
 static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q 
*top_waiter,