[address@hidden: [PULL 1/1] block/nvme: fix infinite loop in nvme_free_req_queue_cb()]

[Stefan Hajnoczi]

Hi,
Please add this to -stable. Thanks!

Stefan

----- Forwarded message from Stefan Hajnoczi <stefanha@redhat.com> -----

Date: Thu,  9 Dec 2021 15:21:17 +0000
From: Stefan Hajnoczi <stefanha@redhat.com>
To: qemu-devel@nongnu.org, Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-block@nongnu.org, Fam Zheng <fam@euphon.net>, Stefan Hajnoczi 
<stefanha@redhat.com>, Hanna Reitz <hreitz@redhat.com>, Paolo Bonzini 
<pbonzini@redhat.com>, Philippe Mathieu-Daudé
        <philmd@redhat.com>, Kevin Wolf <kwolf@redhat.com>
Subject: [PULL 1/1] block/nvme: fix infinite loop in nvme_free_req_queue_cb()
Message-Id: <20211209152117.383832-2-stefanha@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>
---
 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);
 }
-- 
2.33.1


----- End forwarded message -----

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