io_uring: combine lock/unlock sections on exit

io_ring_exit_work() already does uring_lock lock/unlock, no need to
repeat it for lock waiting trick in io_ring_ctx_free(). Move the waiting
with comments and spinlocking into io_ring_exit_work.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a8ae0589b0ea64ad4791e2c282e4e9b713dd7024.1617287883.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8478,16 +8478,6 @@ static void io_req_caches_free(struct io_ring_ctx *ctx)
 
 static void io_ring_ctx_free(struct io_ring_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(). Wait for them to finish.
-	 */
-	mutex_lock(&ctx->uring_lock);
-	mutex_unlock(&ctx->uring_lock);
-	spin_lock_irq(&ctx->completion_lock);
-	spin_unlock_irq(&ctx->completion_lock);
-
 	io_sq_thread_finish(ctx);
 	io_sqe_buffers_unregister(ctx);
 
@@ -8638,6 +8628,12 @@ static void io_ring_exit_work(struct work_struct *work)
 		WARN_ON_ONCE(time_after(jiffies, timeout));
 	} while (!wait_for_completion_timeout(&ctx->ref_comp, HZ/20));
 
+	/*
+	 * 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));
@@ -8658,6 +8654,8 @@ static void io_ring_exit_work(struct work_struct *work)
 		mutex_lock(&ctx->uring_lock);
 	}
 	mutex_unlock(&ctx->uring_lock);
+	spin_lock_irq(&ctx->completion_lock);
+	spin_unlock_irq(&ctx->completion_lock);
 
 	io_ring_ctx_free(ctx);
 }