linux-kernel - [PATCH] io_uring/io-wq: fix `max_workers` breakage and `nr

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Message-ID: <20250912000609.1429966-1-max.kellermann@ionos.com>
Date: Fri, 12 Sep 2025 02:06:09 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: Jens Axboe <axboe@...nel.dk>,
	Fengnan Chang <changfengnan@...edance.com>,
	Sasha Levin <sashal@...nel.org>,
	Diangang Li <lidiangang@...edance.com>,
	io-uring@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Max Kellermann <max.kellermann@...os.com>,
	stable@...r.kernel.org
Subject: [PATCH] io_uring/io-wq: fix `max_workers` breakage and `nr_workers` underflow

Commit 88e6c42e40de ("io_uring/io-wq: add check free worker before
create new worker") reused the variable `do_create` for something
else, abusing it for the free worker check.

This caused the value to effectively always be `true` at the time
`nr_workers < max_workers` was checked, but it should really be
`false`.  This means the `max_workers` setting was ignored, and worse:
if the limit had already been reached, incrementing `nr_workers` was
skipped even though another worker would be created.

When later lots of workers exit, the `nr_workers` field could easily
underflow, making the problem worse because more and more workers
would be created without incrementing `nr_workers`.

The simple solution is to use a different variable for the free worker
check instead of using one variable for two different things.

Cc: stable@...r.kernel.org
Fixes: 88e6c42e40de ("io_uring/io-wq: add check free worker before create new worker")
Signed-off-by: Max Kellermann <max.kellermann@...os.com>
---
 io_uring/io-wq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
index 17dfaa0395c4..1d03b2fc4b25 100644
--- a/io_uring/io-wq.c
+++ b/io_uring/io-wq.c
@@ -352,16 +352,16 @@ static void create_worker_cb(struct callback_head *cb)
 	struct io_wq *wq;

 	struct io_wq_acct *acct;
-	bool do_create = false;
+	bool activated_free_worker, do_create = false;

 	worker = container_of(cb, struct io_worker, create_work);
 	wq = worker->wq;
 	acct = worker->acct;

 	rcu_read_lock();
-	do_create = !io_acct_activate_free_worker(acct);
+	activated_free_worker = io_acct_activate_free_worker(acct);
 	rcu_read_unlock();
-	if (!do_create)
+	if (activated_free_worker)
 		goto no_need_create;

 	raw_spin_lock(&acct->workers_lock);
-- 
2.47.3