> On May 9, 2020, at 12:33 AM, Paul E. McKenney <[email protected]> wrote:
>
> On Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote:
>>
>>
>>> On Feb 11, 2020, at 8:54 AM, Qian Cai <[email protected]> wrote:
>>>
>>> prev->next could 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]>
>>
>> Hmm, this patch has been dropped from linux-next from some reasons.
>>
>> Paul, can you pick this up along with KCSAN fixes?
>>
>> https://lore.kernel.org/lkml/[email protected]/
>
> I have queued it on -rcu, but it is too late for v5.8 via the -rcu tree,
> so this is v5.9 at the earliest. Plus I would need an ack from one of
> the locking folks.
Peter, Will, can you give an ACK? This v2 should incorporate all the feedback
from Peter,
https://lore.kernel.org/lkml/[email protected]/
V5.9 is fine. All I care about is it is always in linux-next (so the testing
bots won’t trigger this over and over again) and to be in mainline at some
point in the future.
>
> Thanx, Paul
>
>>> ---
>>>
>>> v2: insert some code comments.
>>>
>>> kernel/locking/osq_lock.c | 6 +++++-
>>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
>>> index 1f7734949ac8..f733bcd99e8a 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;
>>>
>>> --
>>> 1.8.3.1
