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io-wq: fix wakeup race when adding new work

Browse source 
When new work is added, io_wqe_enqueue() checks if we need to wake or
create a new worker. But that check is done outside the lock that
otherwise synchronizes us with a worker going to sleep, so we can end
up in the following situation:

CPU0				CPU1
lock
insert work
unlock
atomic_read(nr_running) != 0
				lock
				atomic_dec(nr_running)
no wakeup needed

Hold the wqe lock around the "need to wakeup" check. Then we can also get
rid of the temporary work_flags variable, as we know the work will remain
valid as long as we hold the lock.

Cc: stable@vger.kernel.org
Reported-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2021-08-30 07:45:47 -06:00
parent a9a4aa9fbf
commit 87df7fb922
1 changed files with 4 additions and 4 deletions
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8
fs/io-wq.c
View file

@ -794,7 +794,7 @@ static void io_wqe_insert_work(struct io_wqe *wqe, struct io_wq_work *work)
static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
{
struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
int work_flags;
bool do_wake;
/*
* If io-wq is exiting for this task, or if the request has explicitly
@ -806,14 +806,14 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
return;
}
work_flags = work->flags;
raw_spin_lock(&wqe->lock);
io_wqe_insert_work(wqe, work);
wqe->flags &= ~IO_WQE_FLAG_STALLED;
do_wake = (work->flags & IO_WQ_WORK_CONCURRENT) ||
!atomic_read(&acct->nr_running);
raw_spin_unlock(&wqe->lock);
if ((work_flags & IO_WQ_WORK_CONCURRENT) ||
!atomic_read(&acct->nr_running))
if (do_wake)
io_wqe_wake_worker(wqe, acct);
}