[RFC PATCH] io_uring: don't submit sqes when ctx->refs is dying

[Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>]

When IORING_SETUP_SQPOLL is enabled, io_ring_ctx_wait_and_kill() will wait
for sq thread to idle by busy loop:
    while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
        cond_resched();
Above codes are not friendly, indeed I think this busy loop will introduce a
cpu burst in current cpu, though it maybe short.

In this patch, if ctx->refs is dying, we forbids sq_thread from submitting
sqes anymore, just discard leftover sqes.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
---
 fs/io_uring.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ec0aa6957882..6e51140a5722 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5980,7 +5980,8 @@ static int io_sq_thread(void *data)
 		 * If submit got -EBUSY, flag us as needing the application
 		 * to enter the kernel to reap and flush events.
 		 */
-		if (!to_submit || ret == -EBUSY) {
+		if (!to_submit || ret == -EBUSY ||
+		    percpu_ref_is_dying(&ctx->refs)) {
 			/*
 			 * Drop cur_mm before scheduling, we can't hold it for
 			 * long periods (or over schedule()). Do this before
@@ -6027,7 +6028,8 @@ static int io_sq_thread(void *data)
 			smp_mb();
 
 			to_submit = io_sqring_entries(ctx);
-			if (!to_submit || ret == -EBUSY) {
+			if (!to_submit || ret == -EBUSY ||
+			    percpu_ref_is_dying(&ctx->refs)) {
 				if (kthread_should_park()) {
 					finish_wait(&ctx->sqo_wait, &wait);
 					break;
@@ -6051,7 +6053,8 @@ static int io_sq_thread(void *data)
 		}
 
 		mutex_lock(&ctx->uring_lock);
-		ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
+		if (likely(!percpu_ref_is_dying(&ctx->refs)))
+			ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
 		mutex_unlock(&ctx->uring_lock);
 		timeout = jiffies + ctx->sq_thread_idle;
 	}
@@ -7323,16 +7326,6 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 	percpu_ref_kill(&ctx->refs);
 	mutex_unlock(&ctx->uring_lock);
 
-	/*
-	 * Wait for sq thread to idle, if we have one. It won't spin on new
-	 * work after we've killed the ctx ref above. This is important to do
-	 * before we cancel existing commands, as the thread could otherwise
-	 * be queueing new work post that. If that's work we need to cancel,
-	 * it could cause shutdown to hang.
-	 */
-	while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
-		cond_resched();
-
 	io_kill_timeouts(ctx);
 	io_poll_remove_all(ctx);
 
-- 
2.17.2