Callers of mutex_unlock() have to make sure that the mutex stays alive
for the whole duration of the function call. For io_uring that means
that the following pattern is not valid unless we ensure that the
context outlives the mutex_unlock() call.
mutex_lock(&ctx->uring_lock);
req_put(req); // typically via io_req_task_submit()
mutex_unlock(&ctx->uring_lock);
Most contexts are fine: io-wq pins requests, syscalls hold the file,
task works are taking ctx references and so on. However, the task work
fallback path doesn't follow the rule.
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 04fc6c802d ("io_uring: save ctx put/get for task_work submit")
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
io_uring/io_uring.c | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 6212f81ed887..c45951f95946 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -272,6 +272,7 @@ static __cold void io_fallback_req_func(struct work_struct *work)
struct io_kiocb *req, *tmp;
struct io_tw_state ts = { .locked = true, };
+ percpu_ref_get(&ctx->refs);
mutex_lock(&ctx->uring_lock);
llist_for_each_entry_safe(req, tmp, node, io_task_work.node)
req->io_task_work.func(req, &ts);
@@ -279,6 +280,7 @@ static __cold void io_fallback_req_func(struct work_struct *work)
return;
io_submit_flush_completions(ctx);
mutex_unlock(&ctx->uring_lock);
+ percpu_ref_put(&ctx->refs);
}
static int io_alloc_hash_table(struct io_hash_table *table, unsigned bits)
@@ -3145,12 +3147,7 @@ static __cold void io_ring_exit_work(struct work_struct *work)
init_completion(&exit.completion);
init_task_work(&exit.task_work, io_tctx_exit_cb);
exit.ctx = ctx;
- /*
- * Some may use context even when all refs and requests have been put,
- * and they are free to do so while still holding uring_lock or
- * completion_lock, see io_req_task_submit(). Apart from other work,
- * this lock/unlock section also waits them to finish.
- */
+
mutex_lock(&ctx->uring_lock);
while (!list_empty(&ctx->tctx_list)) {
WARN_ON_ONCE(time_after(jiffies, timeout));
--
2.43.0
