From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Wed, 8 Dec 2021 15:22:46 +0000 (+0000)
Subject: block/nvme: fix infinite loop in nvme_free_req_queue_cb()
X-Git-Tag: v7.0.0-rc0~132^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=cf4fbc3030c974fff726756a7ceef8386cdf500b;p=thirdparty%2Fqemu.git

block/nvme: fix infinite loop in nvme_free_req_queue_cb()

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>
---

diff --git a/block/nvme.c b/block/nvme.c
index e4f336d79c2..fa360b9b3cc 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);
 }