Re: [PATCH] block/nvme: fix infinite loop in nvme_free_req_queue_cb()

[Philippe Mathieu-Daudé]

On 12/8/21 16:22, Stefan Hajnoczi wrote:
> 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

Ouch.

>     }
> 
> 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>
> ---
>  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 */

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

>      }
>      qemu_mutex_unlock(&q->lock);
>  }
>