Re: [PATCH RFC -next] sbitmap: fix possible io hung due to lost wakeups

From: Yu Kuai
Date: Tue Jun 21 2022 - 23:58:37 EST

在 2022/06/21 1:02, Jan Kara 写道:
On Mon 20-06-22 21:44:16, Yu Kuai wrote:
在 2022/06/20 20:48, Jan Kara 写道:
On Mon 20-06-22 14:24:13, Jan Kara wrote:
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@xxxxxxxxxx>

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...
Hi, Jan

Thanks for the analysis, it's right this is possible.

Thinking a bit more about it your code would just need a small tweak like:

wait_cnt = atomic_dec_return(&ws->wait_cnt);
/*
* Concurrent callers should call this function again
* to wakeup a new batch on a different 'ws'.
*/
if (wait_cnt < 0 || !waitqueue_active(&ws->wait)) {
sbq_update_wake_index(sbq, wake_index);
return true;
}

I'm thinking that if the wait_queue is still active, this will decrease
'wait_cnt' in old waitqueue while 'wake_index' is already moved to next
waitqueue. This really broke the design...

I agree this can happen and it is not ideal. On the other hand the wakeup
is not really lost, just effectively delayed until we select this waitqueue
again so it should not result in any hangs. And other ways to avoid the
race seem more expensive to me...

Hi, Jan

Before you reviewed this version, I aready posted v2... It semms v2 is
using exactly the same logic that you suggested here 😉.

Thanks,
Kuai

Honza

if (wait_cnt > 0)
return false;
sbq_update_wake_index(sbq, wake_index);

wake_batch = READ_ONCE(sbq->wake_batch);
wake_up_nr(&ws->wait, 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.
*
* Also pairs with the implicit barrier between decrementing
* wait_cnt and checking for waitqueue_active() to make sure
* waitqueue_active() sees results of the wakeup if
* atomic_dec_return() has seen results of the atomic_set.
*/
smp_mb__before_atomic();
atomic_set(&ws->wait_cnt, wake_batch);

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@xxxxxxxx>
SUSE Labs, CR