On Sun 10-07-22 12:21:58, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> __sbq_wake_up __sbq_wake_up
> sbq_wake_ptr -> assume 0
> sbq_wake_ptr -> 0
> atomic_dec_return
> atomic_dec_return
> atomic_cmpxchg -> succeed
> atomic_cmpxchg -> failed
> return true
>
> __sbq_wake_up
> sbq_wake_ptr
> atomic_read(&sbq->wake_index) -> still 0
> sbq_index_atomic_inc -> inc to 1
> if (waitqueue_active(&ws->wait))
> if (wake_index != atomic_read(&sbq->wake_index))
> atomic_set -> reset from 1 to 0
> wake_up_nr -> wake up first waitqueue
> // continue to wake up in first waitqueue
>
> Fix the problem by using atomic_cmpxchg() instead of atomic_set()
> to update 'wake_index'.
>
> Fixes: 417232880c8a ("sbitmap: Replace cmpxchg with xchg")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
I don't think this patch is really needed after the following patches. As
I see it, wake_index is just a performance optimization (plus a fairness
improvement) but in principle the code in sbq_wake_ptr() is always prone to
races as the waitqueue it returns needn't have any waiters by the time we
return. So for correctness the check-and-retry loop needs to happen at
higher level than inside sbq_wake_ptr() and occasional wrong setting of
wake_index will result only in a bit of unfairness and more scanning
looking for suitable waitqueue but I don't think that really justifies the
cost of atomic operations in cmpxchg loop...
Honza
> ---
> lib/sbitmap.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 29eb0484215a..b46fce1beb3a 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -579,19 +579,24 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>
> static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> {
> - int i, wake_index;
> + int i, wake_index, old_wake_index;
>
> +again:
> if (!atomic_read(&sbq->ws_active))
> return NULL;
>
> - wake_index = atomic_read(&sbq->wake_index);
> + old_wake_index = wake_index = atomic_read(&sbq->wake_index);
> for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> struct sbq_wait_state *ws = &sbq->ws[wake_index];
>
> if (waitqueue_active(&ws->wait)) {
> - if (wake_index != atomic_read(&sbq->wake_index))
> - atomic_set(&sbq->wake_index, wake_index);
> - return ws;
> + if (wake_index == old_wake_index)
> + return ws;
> +
> + if (atomic_cmpxchg(&sbq->wake_index, old_wake_index,
> + wake_index) == old_wake_index)
> + return ws;
> + goto again;
> }
>
> wake_index = sbq_index_inc(wake_index);
> --
> 2.31.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
