6.16-stable review patch. If anyone has any objections, please let me know.
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From: Fengnan Chang changfengnan@bytedance.com
[ Upstream commit 9d83e1f05c98bab5de350bef89177e2be8b34db0 ]
After commit 0b2b066f8a85 ("io_uring/io-wq: only create a new worker if it can make progress"), in our produce environment, we still observe that part of io_worker threads keeps creating and destroying. After analysis, it was confirmed that this was due to a more complex scenario involving a large number of fsync operations, which can be abstracted as frequent write + fsync operations on multiple files in a single uring instance. Since write is a hash operation while fsync is not, and fsync is likely to be suspended during execution, the action of checking the hash value in io_wqe_dec_running cannot handle such scenarios. Similarly, if hash-based work and non-hash-based work are sent at the same time, similar issues are likely to occur. Returning to the starting point of the issue, when a new work arrives, io_wq_enqueue may wake up free worker A, while io_wq_dec_running may create worker B. Ultimately, only one of A and B can obtain and process the task, leaving the other in an idle state. In the end, the issue is caused by inconsistent logic in the checks performed by io_wq_enqueue and io_wq_dec_running. Therefore, the problem can be resolved by checking for available workers in io_wq_dec_running.
Signed-off-by: Fengnan Chang changfengnan@bytedance.com Reviewed-by: Diangang Li lidiangang@bytedance.com Link: https://lore.kernel.org/r/20250813120214.18729-1-changfengnan@bytedance.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/io-wq.c | 8 ++++++++ 1 file changed, 8 insertions(+)
--- a/io_uring/io-wq.c +++ b/io_uring/io-wq.c @@ -357,6 +357,13 @@ static void create_worker_cb(struct call 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); + rcu_read_unlock(); + if (!do_create) + goto no_need_create; + raw_spin_lock(&acct->workers_lock);
if (acct->nr_workers < acct->max_workers) { @@ -367,6 +374,7 @@ static void create_worker_cb(struct call if (do_create) { create_io_worker(wq, acct); } else { +no_need_create: atomic_dec(&acct->nr_running); io_worker_ref_put(wq); }