> 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]/
> ---
>
> 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
>
