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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Andres Freund <andres@anarazel.de>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.10 006/306] io-wq: fix wakeup race when adding new work
Date:   Thu, 16 Sep 2021 17:55:51 +0200
Message-Id: <20210916155754.130352649@linuxfoundation.org>
In-Reply-To: <20210916155753.903069397@linuxfoundation.org>
References: <20210916155753.903069397@linuxfoundation.org>
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From: Jens Axboe <axboe@kernel.dk>

commit 87df7fb922d18e96992aa5e824aa34b2065fef59 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/io-wq.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -895,7 +895,7 @@ append:
 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;
 	unsigned long flags;
 
 	/*
@@ -909,14 +909,14 @@ static void io_wqe_enqueue(struct io_wqe
 		return;
 	}
 
-	work_flags = work->flags;
 	raw_spin_lock_irqsave(&wqe->lock, flags);
 	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_irqrestore(&wqe->lock, flags);
 
-	if ((work_flags & IO_WQ_WORK_CONCURRENT) ||
-	    !atomic_read(&acct->nr_running))
+	if (do_wake)
 		io_wqe_wake_worker(wqe, acct);
 }