| Message ID | 20220710042200.20936-3-yukuai1@huaweicloud.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | bugfix for sbitmap | expand |
On Sun 10-07-22 12:21:59, Yu Kuai wrote: > From: Yu Kuai <yukuai3@huawei.com> > > For example, 2 * wake_batch tags are put, while only wake_batch threads > are woken: > > __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, increasing 'wake_index' before resetting 'wait_cnt'. > > Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library") > Signed-off-by: Yu Kuai <yukuai3@huawei.com> This patch and the following one look sane to me but please merge them to a single patch. They fix the same race of two concurrent wakers just with a slightly different timing so there isn't a point in having two patches for this (in particular changes in this patch are difficult to reason about when we know the result is still buggy). Honza > --- > lib/sbitmap.c | 45 +++++++++++++++++++++++---------------------- > 1 file changed, 23 insertions(+), 22 deletions(-) > > diff --git a/lib/sbitmap.c b/lib/sbitmap.c > index b46fce1beb3a..57095dd88a33 100644 > --- a/lib/sbitmap.c > +++ b/lib/sbitmap.c > @@ -616,32 +616,33 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq) > return false; > > wait_cnt = atomic_dec_return(&ws->wait_cnt); > - if (wait_cnt <= 0) { > - int ret; > + if (wait_cnt > 0) > + return false; > > - wake_batch = READ_ONCE(sbq->wake_batch); > + /* > + * For concurrent callers of this, callers should call this function > + * again to wakeup a new batch on a different 'ws'. > + */ > + if (wait_cnt < 0) > + return true; > > - /* > - * 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_batch = READ_ONCE(sbq->wake_batch); > > - /* > - * 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; > - } > + /* > + * 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(); > > - return true; > - } > + /* > + * Increase wake_index before updating wait_cnt, otherwise concurrent > + * callers can see valid wait_cnt in old waitqueue, which can cause > + * invalid wakeup on the old waitqueue. > + */ > + sbq_index_atomic_inc(&sbq->wake_index); > + atomic_set(&ws->wait_cnt, wake_batch); > + wake_up_nr(&ws->wait, wake_batch); > > return false; > } > -- > 2.31.1 >
Hi! 在 2022/07/11 22:26, Jan Kara 写道: > On Sun 10-07-22 12:21:59, Yu Kuai wrote: >> From: Yu Kuai <yukuai3@huawei.com> >> >> For example, 2 * wake_batch tags are put, while only wake_batch threads >> are woken: >> >> __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, increasing 'wake_index' before resetting 'wait_cnt'. >> >> Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library") >> Signed-off-by: Yu Kuai <yukuai3@huawei.com> > > This patch and the following one look sane to me but please merge them to a > single patch. They fix the same race of two concurrent wakers just with a > slightly different timing so there isn't a point in having two patches for > this (in particular changes in this patch are difficult to reason about > when we know the result is still buggy). Ok, I'll merge them. Thanks, Kuai > > Honza > >> --- >> lib/sbitmap.c | 45 +++++++++++++++++++++++---------------------- >> 1 file changed, 23 insertions(+), 22 deletions(-) >> >> diff --git a/lib/sbitmap.c b/lib/sbitmap.c >> index b46fce1beb3a..57095dd88a33 100644 >> --- a/lib/sbitmap.c >> +++ b/lib/sbitmap.c >> @@ -616,32 +616,33 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq) >> return false; >> >> wait_cnt = atomic_dec_return(&ws->wait_cnt); >> - if (wait_cnt <= 0) { >> - int ret; >> + if (wait_cnt > 0) >> + return false; >> >> - wake_batch = READ_ONCE(sbq->wake_batch); >> + /* >> + * For concurrent callers of this, callers should call this function >> + * again to wakeup a new batch on a different 'ws'. >> + */ >> + if (wait_cnt < 0) >> + return true; >> >> - /* >> - * 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_batch = READ_ONCE(sbq->wake_batch); >> >> - /* >> - * 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; >> - } >> + /* >> + * 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(); >> >> - return true; >> - } >> + /* >> + * Increase wake_index before updating wait_cnt, otherwise concurrent >> + * callers can see valid wait_cnt in old waitqueue, which can cause >> + * invalid wakeup on the old waitqueue. >> + */ >> + sbq_index_atomic_inc(&sbq->wake_index); >> + atomic_set(&ws->wait_cnt, wake_batch); >> + wake_up_nr(&ws->wait, wake_batch); >> >> return false; >> } >> -- >> 2.31.1 >>
diff --git a/lib/sbitmap.c b/lib/sbitmap.c index b46fce1beb3a..57095dd88a33 100644 --- a/lib/sbitmap.c +++ b/lib/sbitmap.c @@ -616,32 +616,33 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq) return false; wait_cnt = atomic_dec_return(&ws->wait_cnt); - if (wait_cnt <= 0) { - int ret; + if (wait_cnt > 0) + return false; - wake_batch = READ_ONCE(sbq->wake_batch); + /* + * For concurrent callers of this, callers should call this function + * again to wakeup a new batch on a different 'ws'. + */ + if (wait_cnt < 0) + return true; - /* - * 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_batch = READ_ONCE(sbq->wake_batch); - /* - * 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; - } + /* + * 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(); - return true; - } + /* + * Increase wake_index before updating wait_cnt, otherwise concurrent + * callers can see valid wait_cnt in old waitqueue, which can cause + * invalid wakeup on the old waitqueue. + */ + sbq_index_atomic_inc(&sbq->wake_index); + atomic_set(&ws->wait_cnt, wake_batch); + wake_up_nr(&ws->wait, wake_batch); return false; }