io_uring/io-wq: limit retrying worker initialisation

commit 0453aad676ff99787124b9b3af4a5f59fbe808e2 upstream.

If io-wq worker creation fails, we retry it by queueing up a task_work.
tasK_work is needed because it should be done from the user process
context. The problem is that retries are not limited, and if queueing a
task_work is the reason for the failure, we might get into an infinite
loop.

It doesn't seem to happen now but it would with the following patch
executing task_work in the freezer's loop. For now, arbitrarily limit the
number of attempts to create a worker.

Cc: stable@vger.kernel.org
Fixes: 3146cba99a ("io-wq: make worker creation resilient against signals")
Reported-by: Julian Orth <ju.orth@gmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8280436925db88448c7c85c6656edee1a43029ea.1720634146.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

io_uring/io-wq.c

@@ -19,6 +19,7 @@
 #include "io-wq.h"
 
 #define WORKER_IDLE_TIMEOUT	(5 * HZ)
+#define WORKER_INIT_LIMIT	3
 
 enum {
 	IO_WORKER_F_UP		= 1,	/* up and active */
@@ -54,6 +55,7 @@ struct io_worker {
 	unsigned long create_state;
 	struct callback_head create_work;
 	int create_index;
+	int init_retries;
 
 	union {
 		struct rcu_head rcu;
@@ -732,7 +734,7 @@ static bool io_wq_work_match_all(struct io_wq_work *work, void *data)
 	return true;
 }
 
-static inline bool io_should_retry_thread(long err)
+static inline bool io_should_retry_thread(struct io_worker *worker, long err)
 {
 	/*
 	 * Prevent perpetual task_work retry, if the task (or its group) is
@@ -740,6 +742,8 @@ static inline bool io_should_retry_thread(long err)
 	 */
 	if (fatal_signal_pending(current))
 		return false;
+	if (worker->init_retries++ >= WORKER_INIT_LIMIT)
+		return false;
 
 	switch (err) {
 	case -EAGAIN:
@@ -766,7 +770,7 @@ static void create_worker_cont(struct callback_head *cb)
 		io_init_new_worker(wqe, worker, tsk);
 		io_worker_release(worker);
 		return;
-	} else if (!io_should_retry_thread(PTR_ERR(tsk))) {
+	} else if (!io_should_retry_thread(worker, PTR_ERR(tsk))) {
 		struct io_wqe_acct *acct = io_wqe_get_acct(worker);
 
 		atomic_dec(&acct->nr_running);
@@ -831,7 +835,7 @@ fail:
 	tsk = create_io_thread(io_wqe_worker, worker, wqe->node);
 	if (!IS_ERR(tsk)) {
 		io_init_new_worker(wqe, worker, tsk);
-	} else if (!io_should_retry_thread(PTR_ERR(tsk))) {
+	} else if (!io_should_retry_thread(worker, PTR_ERR(tsk))) {
 		kfree(worker);
 		goto fail;
 	} else {