lkml.org 
[lkml]   [2017]   [Aug]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] rwsem: fix missed wakeup due to reordering of load
On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote:
>
> WTH did you not Cc the people that commented on your patch last time?
>
> On Wed, Aug 23, 2017 at 04:58:55PM +0530, Prateek Sood wrote:
> > If a spinner is present, there is a chance that the load of
> > rwsem_has_spinner() in rwsem_wake() can be reordered with
> > respect to decrement of rwsem count in __up_write() leading
> > to wakeup being missed.
>
> > spinning writer up_write caller
> > --------------- -----------------------
> > [S] osq_unlock() [L] osq
> > spin_lock(wait_lock)
> > sem->count=0xFFFFFFFF00000001
> > +0xFFFFFFFF00000000
> > count=sem->count
> > MB
> > sem->count=0xFFFFFFFE00000001
> > -0xFFFFFFFF00000001
> > RMB
>
> This doesn't make sense, it appears to order a STORE against something
> else.
>
> > spin_trylock(wait_lock)
> > return
> > rwsem_try_write_lock(count)
> > spin_unlock(wait_lock)
> > schedule()

Is this what you wanted to write?

---
kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f660666ab8..813b5d3654ce 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
DEFINE_WAKE_Q(wake_q);

/*
+ * __rwsem_down_write_failed_common(sem)
+ * rwsem_optimistic_spin(sem)
+ * osq_unlock(sem->osq)
+ * ...
+ * atomic_long_add_return(&sem->count)
+ *
+ * - VS -
+ *
+ * __up_write()
+ * if (atomic_long_sub_return_release(&sem->count) < 0)
+ * rwsem_wake(sem)
+ * osq_is_locked(&sem->osq)
+ *
+ * And __up_write() must observe !osq_is_locked() when it observes the
+ * atomic_long_add_return() in order to not miss a wakeup.
+ *
+ * This boils down to:
+ *
+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
+ * MB RMB
+ * [RmW] Y += 1 [L] r1 = X
+ *
+ * exists (r0=1 /\ r1=0)
+ */
+ smp_rmb();
+
+ /*
* If a spinner is present, it is not necessary to do the wakeup.
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the
\
 
 \ /
  Last update: 2017-08-24 14:33    [W:0.056 / U:42.100 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site