From 6989be9d0aa08470f8b287c243dc4bf027d5fbcf Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Wed, 8 Dec 2021 15:22:46 +0000
Subject: [PATCH 1/2] block/nvme: fix infinite loop in nvme_free_req_queue_cb()
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
RH-MergeRequest: 58: block/nvme: fix infinite loop in nvme_free_req_queue_cb()
RH-Commit: [1/1] 544b3f310d791a20c63b51947de0c6cbb60b0d5b (stefanha/centos-stream-qemu-kvm)
RH-Bugzilla: 2024544
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
RH-Acked-by: Hanna Reitz <hreitz@redhat.com>

When the request free list is exhausted the coroutine waits on
q->free_req_queue for the next free request. Whenever a request is
completed a BH is scheduled to invoke nvme_free_req_queue_cb() and wake
up waiting coroutines.

1. nvme_get_free_req() waits for a free request:

    while (q->free_req_head == -1) {
        ...
            trace_nvme_free_req_queue_wait(q->s, q->index);
            qemu_co_queue_wait(&q->free_req_queue, &q->lock);
        ...
    }

2. nvme_free_req_queue_cb() wakes up the coroutine:

    while (qemu_co_enter_next(&q->free_req_queue, &q->lock)) {
       ^--- infinite loop when free_req_head == -1
    }

nvme_free_req_queue_cb() and the coroutine form an infinite loop when
q->free_req_head == -1. Fix this by checking q->free_req_head in
nvme_free_req_queue_cb(). If the free request list is exhausted, don't
wake waiting coroutines. Eventually an in-flight request will complete
and the BH will be scheduled again, guaranteeing forward progress.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20211208152246.244585-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit cf4fbc3030c974fff726756a7ceef8386cdf500b)
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/nvme.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/nvme.c b/block/nvme.c
index e4f336d79c..fa360b9b3c 100644
--- a/block/nvme.c
+++ b/block/nvme.c
@@ -206,8 +206,9 @@ static void nvme_free_req_queue_cb(void *opaque)
     NVMeQueuePair *q = opaque;
 
     qemu_mutex_lock(&q->lock);
-    while (qemu_co_enter_next(&q->free_req_queue, &q->lock)) {
-        /* Retry all pending requests */
+    while (q->free_req_head != -1 &&
+           qemu_co_enter_next(&q->free_req_queue, &q->lock)) {
+        /* Retry waiting requests */
     }
     qemu_mutex_unlock(&q->lock);
 }
-- 
2.27.0