FAILED: patch "[PATCH] io-wq: fix wakeup race when adding new work" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 87df7fb922d18e96992aa5e824aa34b2065fef59 Mon Sep 17 00:00:00 2001
From: Jens Axboe <[email protected]>
Date: Mon, 30 Aug 2021 07:45:47 -0600
Subject: [PATCH] io-wq: fix wakeup race when adding new work

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.

index 13aeb48a0964..cd9bd095fb1b 100644
@@ -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);
 }