On Sat, May 09, 2020 at 12:53:38PM -0400, Qian Cai wrote:
>
>
> > On May 9, 2020, at 12:12 PM, Paul E. McKenney <[email protected]> wrote:
> >
> > Ah, and I forgot to ask. Why "if (data_race(prev->next == node)" instead
> > of "if (data_race(prev->next) == node"?
>
> I think the one you suggested is slightly better to point out the exact race.
> Do you want me to resend or you could squash it instead?
The patch was still at the top of my stack, so I just amended it. Here
is the updated version.
Thanx, Paul
------------------------------------------------------------------------
commit 13e69ca01ce1621ce74248bda86cfad47fa5a0fa
Author: Qian Cai <[email protected]>
Date: Tue Feb 11 08:54:15 2020 -0500
locking/osq_lock: Annotate a data race in osq_lock
The prev->next pointer can be accessed concurrently as noticed by KCSAN:
write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
osq_lock+0x25f/0x350
osq_wait_next at kernel/locking/osq_lock.c:79
(inlined by) osq_lock at kernel/locking/osq_lock.c:185
rwsem_optimistic_spin
<snip>
read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
osq_lock+0x196/0x350
osq_lock at kernel/locking/osq_lock.c:157
rwsem_optimistic_spin
<snip>
Since the write only stores NULL to prev->next and the read tests if
prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
shattered, the code is still working correctly. Thus, mark it as an
intentional data race using the data_race() macro.
Signed-off-by: Qian Cai <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 1f77349..1de006e 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
*/
for (;;) {
- if (prev->next == node &&
+ /*
+ * cpu_relax() below implies a compiler barrier which would
+ * prevent this comparison being optimized away.
+ */
+ if (data_race(prev->next) == node &&
cmpxchg(&prev->next, node, NULL) == node)
break;
