FAILED: patch "[PATCH] io-wq: fix race between adding work and activating a free" failed to apply to 5.10-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

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 <stable@xxxxxxxxxxxxxxx>.

thanks,

greg k-h

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

>From 94ffb0a282872c2f4b14f757fa1aef2302aeaabb Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@xxxxxxxxx>
Date: Mon, 30 Aug 2021 11:55:22 -0600
Subject: [PATCH] io-wq: fix race between adding work and activating a free
 worker

The attempt to find and activate a free worker for new work is currently
combined with creating a new one if we don't find one, but that opens
io-wq up to a race where the worker that is found and activated can
put itself to sleep without knowing that it has been selected to perform
this new work.

Fix this by moving the activation into where we add the new work item,
then we can retain it within the wqe->lock scope and elimiate the race
with the worker itself checking inside the lock, but sleeping outside of
it.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Andres Freund <andres@xxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

diff --git a/fs/io-wq.c b/fs/io-wq.c
index cd9bd095fb1b..94f8f2ecb8e5 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -236,9 +236,9 @@ static bool io_wqe_activate_free_worker(struct io_wqe *wqe)
  * We need a worker. If we find a free one, we're good. If not, and we're
  * below the max number of workers, create one.
  */
-static void io_wqe_wake_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
+static void io_wqe_create_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
 {
-	bool ret;
+	bool do_create = false, first = false;
 
 	/*
 	 * Most likely an attempt to queue unbounded work on an io_wq that
@@ -247,26 +247,18 @@ static void io_wqe_wake_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
 	if (unlikely(!acct->max_workers))
 		pr_warn_once("io-wq is not configured for unbound workers");
 
-	rcu_read_lock();
-	ret = io_wqe_activate_free_worker(wqe);
-	rcu_read_unlock();
-
-	if (!ret) {
-		bool do_create = false, first = false;
-
-		raw_spin_lock(&wqe->lock);
-		if (acct->nr_workers < acct->max_workers) {
-			if (!acct->nr_workers)
-				first = true;
-			acct->nr_workers++;
-			do_create = true;
-		}
-		raw_spin_unlock(&wqe->lock);
-		if (do_create) {
-			atomic_inc(&acct->nr_running);
-			atomic_inc(&wqe->wq->worker_refs);
-			create_io_worker(wqe->wq, wqe, acct->index, first);
-		}
+	raw_spin_lock(&wqe->lock);
+	if (acct->nr_workers < acct->max_workers) {
+		if (!acct->nr_workers)
+			first = true;
+		acct->nr_workers++;
+		do_create = true;
+	}
+	raw_spin_unlock(&wqe->lock);
+	if (do_create) {
+		atomic_inc(&acct->nr_running);
+		atomic_inc(&wqe->wq->worker_refs);
+		create_io_worker(wqe->wq, wqe, acct->index, first);
 	}
 }
 
@@ -794,7 +786,8 @@ 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);
-	bool do_wake;
+	unsigned work_flags = work->flags;
+	bool do_create;
 
 	/*
 	 * If io-wq is exiting for this task, or if the request has explicitly
@@ -809,12 +802,16 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
 	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);
+
+	rcu_read_lock();
+	do_create = !io_wqe_activate_free_worker(wqe);
+	rcu_read_unlock();
+
 	raw_spin_unlock(&wqe->lock);
 
-	if (do_wake)
-		io_wqe_wake_worker(wqe, acct);
+	if (do_create && ((work_flags & IO_WQ_WORK_CONCURRENT) ||
+	    !atomic_read(&acct->nr_running)))
+		io_wqe_create_worker(wqe, acct);
 }
 
 void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work)