io_uring/io-wq: fix max_workers breakage and nr_workers underflow

Commit 88e6c42 ("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@vger.kernel.org
Fixes: 88e6c42 ("io_uring/io-wq: add check free worker before create new worker")
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Fengnan Chang <changfengnan@bytedance.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: MaxKellermann

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);