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Message-ID: <20220620122413.2fewshn5u6t2y4oi@quack3.lan>
Date: Mon, 20 Jun 2022 14:24:13 +0200
From: Jan Kara <jack@...e.cz>
To: Yu Kuai <yukuai3@...wei.com>
Cc: axboe@...nel.dk, ming.lei@...hat.com, jack@...e.cz,
linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
yi.zhang@...wei.com
Subject: Re: [PATCH RFC -next] sbitmap: fix possible io hung due to lost
wakeups
On Fri 17-06-22 22:11:25, Yu Kuai wrote:
> Currently, same waitqueue might be woken up continuously:
>
> __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
>
> What's worse, io hung is possible in theory because wakeups might be
> missed. For example, 2 * wake_batch tags are put, while only wake_batch
> threads are worken:
>
> __sbq_wake_up
> atomic_cmpxchg -> reset wait_cnt
> __sbq_wake_up -> decrease wait_cnt
> ...
> __sbq_wake_up -> wait_cnt is decreased to 0 again
> atomic_cmpxchg
> sbq_index_atomic_inc -> increase wake_index
> wake_up_nr -> wake up and waitqueue might be empty
> sbq_index_atomic_inc -> increase again, one waitqueue is skipped
> wake_up_nr -> invalid wake up because old wakequeue might be empty
>
> To fix the problem, refactor to make sure waitqueues will be woken up
> one by one,
>
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
So as far as I can tell your patch does not completely fix this race. See
below:
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index ae4fd4de9ebe..dc2959cb188c 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -574,66 +574,69 @@ void sbitmap_queue_min_shallow_depth(struct sbitmap_queue *sbq,
> }
> EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>
> -static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> +static void sbq_update_wake_index(struct sbitmap_queue *sbq,
> + int old_wake_index)
> {
> int i, wake_index;
> -
> - if (!atomic_read(&sbq->ws_active))
> - return NULL;
> + struct sbq_wait_state *ws;
>
> 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 (old_wake_index != wake_index)
> + return;
>
> + for (i = 1; i < SBQ_WAIT_QUEUES; i++) {
> + wake_index = sbq_index_inc(wake_index);
> + ws = &sbq->ws[wake_index];
> + /* Find the next active waitqueue in round robin manner */
> if (waitqueue_active(&ws->wait)) {
> - if (wake_index != atomic_read(&sbq->wake_index))
> - atomic_set(&sbq->wake_index, wake_index);
> - return ws;
> + atomic_cmpxchg(&sbq->wake_index, old_wake_index,
> + wake_index);
> + return;
> }
> -
> - wake_index = sbq_index_inc(wake_index);
> }
> -
> - return NULL;
> }
>
> static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> {
> struct sbq_wait_state *ws;
> unsigned int wake_batch;
> - int wait_cnt;
> + int wait_cnt, wake_index;
>
> - ws = sbq_wake_ptr(sbq);
> - if (!ws)
> + if (!atomic_read(&sbq->ws_active))
> return false;
>
> - wait_cnt = atomic_dec_return(&ws->wait_cnt);
> - if (wait_cnt <= 0) {
> - int ret;
> -
> - wake_batch = READ_ONCE(sbq->wake_batch);
> -
> - /*
> - * Pairs with the memory barrier in sbitmap_queue_resize() to
> - * ensure that we see the batch size update before the wait
> - * count is reset.
> - */
> - smp_mb__before_atomic();
> + wake_index = atomic_read(&sbq->wake_index);
> + ws = &sbq->ws[wake_index];
> + /*
> + * This can only happen in the first wakeup when sbitmap waitqueues
> + * are no longer idle.
> + */
> + if (!waitqueue_active(&ws->wait)) {
> + sbq_update_wake_index(sbq, wake_index);
> + return true;
> + }
>
> - /*
> - * For concurrent callers of this, the one that failed the
> - * atomic_cmpxhcg() race should call this function again
> - * to wakeup a new batch on a different 'ws'.
> - */
> - ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
> - if (ret == wait_cnt) {
> - sbq_index_atomic_inc(&sbq->wake_index);
> - wake_up_nr(&ws->wait, wake_batch);
> - return false;
> - }
> + wait_cnt = atomic_dec_return(&ws->wait_cnt);
> + if (wait_cnt > 0)
> + return false;
The following race is still possible:
CPU1 CPU2
__sbq_wake_up __sbq_wake_up
wake_index = atomic_read(&sbq->wake_index);
wake_index = atomic_read(&sbq->wake_index);
if (!waitqueue_active(&ws->wait)) -> not taken
if (!waitqueue_active(&ws->wait)) -> not taken
wait_cnt = atomic_dec_return(&ws->wait_cnt);
/* decremented to 0 now */
if (wait_cnt > 0) -> not taken
sbq_update_wake_index(sbq, wake_index);
if (wait_cnt < 0) -> not taken
...
atomic_set(&ws->wait_cnt, wake_batch);
wake_up_nr(&ws->wait, wake_batch);
wait_cnt = atomic_dec_return(&ws->wait_cnt);
/*
* decremented to wake_batch - 1 but
* there are no tasks waiting anymore
* so the wakeup should have gone
* to a different waitqueue.
*/
I have an idea how to fix all these lost wakeups, I'll try to code it
whether it would look usable...
Honza
> + sbq_update_wake_index(sbq, wake_index);
> + /*
> + * Concurrent callers should call this function again
> + * to wakeup a new batch on a different 'ws'.
> + */
> + if (wait_cnt < 0)
> return true;
> - }
> +
> + wake_batch = READ_ONCE(sbq->wake_batch);
> + /*
> + * Pairs with the memory barrier in sbitmap_queue_resize() to
> + * ensure that we see the batch size update before the wait
> + * count is reset.
> + */
> + smp_mb__before_atomic();
> + atomic_set(&ws->wait_cnt, wake_batch);
> + wake_up_nr(&ws->wait, wake_batch);
>
> return false;
> }
> --
> 2.31.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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