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) <[email protected]>
---
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(&q->lock_ptr, NULL);
}
static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
On Mon, 03 Oct 2016 11:12:36 +0200
Peter Zijlstra <[email protected]> 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) <[email protected]>
> ---
> 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(&q->lock_ptr, NULL);
> }
>
> static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
>
Reviewed-by: Steven Rostedt <[email protected]>
-- Steve
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) <[email protected]>
>---
> 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(&q->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
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) <[email protected]>
> >---
> >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(&q->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.