FAILED: patch "[PATCH] io_uring: don't keep looping for more events if we can't" failed to apply to 5.11-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.11-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ca0a26511c679a797f86589894a4523db36d833e Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Thu, 4 Mar 2021 17:15:48 -0700
Subject: [PATCH] io_uring: don't keep looping for more events if we can't
 flush overflow

It doesn't make sense to wait for more events to come in, if we can't
even flush the overflow we already have to the ring. Return -EBUSY for
that condition, just like we do for attempts to submit with overflow
pending.

Cc: stable@vger.kernel.org # 5.11
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 76e243c056b5..044170165402 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1451,18 +1451,22 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
 	return all_flushed;
 }
 
-static void io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
+static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
 				     struct task_struct *tsk,
 				     struct files_struct *files)
 {
+	bool ret = true;
+
 	if (test_bit(0, &ctx->cq_check_overflow)) {
 		/* iopoll syncs against uring_lock, not completion_lock */
 		if (ctx->flags & IORING_SETUP_IOPOLL)
 			mutex_lock(&ctx->uring_lock);
-		__io_cqring_overflow_flush(ctx, force, tsk, files);
+		ret = __io_cqring_overflow_flush(ctx, force, tsk, files);
 		if (ctx->flags & IORING_SETUP_IOPOLL)
 			mutex_unlock(&ctx->uring_lock);
 	}
+
+	return ret;
 }
 
 static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
@@ -6883,11 +6887,16 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
 	trace_io_uring_cqring_wait(ctx, min_events);
 	do {
-		io_cqring_overflow_flush(ctx, false, NULL, NULL);
+		/* if we can't even flush overflow, don't wait for more */
+		if (!io_cqring_overflow_flush(ctx, false, NULL, NULL)) {
+			ret = -EBUSY;
+			break;
+		}
 		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
 						TASK_INTERRUPTIBLE);
 		ret = io_cqring_wait_schedule(ctx, &iowq, &timeout);
 		finish_wait(&ctx->wait, &iowq.wq);
+		cond_resched();
 	} while (ret > 0);
 
 	restore_saved_sigmask_unless(ret == -EINTR);