Re: [PATCH 3/7] block/nvme: don't access CQE after moving cq.head

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 5/19/20 7:11 PM, Stefan Hajnoczi wrote:
> Do not access a CQE after incrementing q->cq.head and releasing q->lock.
> It is unlikely that this causes problems in practice but it's a latent
> bug.
> 
> The reason why it should be safe at the moment is that completion
> processing is not re-entrant and the CQ doorbell isn't written until the
> end of nvme_process_completion().
> 
> Make this change now because QEMU expects completion processing to be
> re-entrant and later patches will do that.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/nvme.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/block/nvme.c b/block/nvme.c
> index 5286227074..6bf58bc6aa 100644
> --- a/block/nvme.c
> +++ b/block/nvme.c
> @@ -321,11 +321,14 @@ static bool nvme_process_completion(BDRVNVMeState *s, NVMeQueuePair *q)
>      q->busy = true;
>      assert(q->inflight >= 0);
>      while (q->inflight) {
> +        int ret;
>          int16_t cid;
> +
>          c = (NvmeCqe *)&q->cq.queue[q->cq.head * NVME_CQ_ENTRY_BYTES];
>          if ((le16_to_cpu(c->status) & 0x1) == q->cq_phase) {
>              break;
>          }
> +        ret = nvme_translate_error(c);

Tricky.

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

>          q->cq.head = (q->cq.head + 1) % NVME_QUEUE_SIZE;
>          if (!q->cq.head) {
>              q->cq_phase = !q->cq_phase;
> @@ -344,7 +347,7 @@ static bool nvme_process_completion(BDRVNVMeState *s, NVMeQueuePair *q)
>          preq->busy = false;
>          preq->cb = preq->opaque = NULL;
>          qemu_mutex_unlock(&q->lock);
> -        req.cb(req.opaque, nvme_translate_error(c));
> +        req.cb(req.opaque, ret);
>          qemu_mutex_lock(&q->lock);
>          q->inflight--;
>          progress = true;
>