Messages in this thread | | | Subject | Re: [V9fs-developer] [PATCH] net/9p: Fix a deadlock case in the virtio transport | From | jiangyiwen <> | Date | Sat, 14 Jul 2018 19:12:37 +0800 |
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On 2018/7/14 17:05, Dominique Martinet wrote: > jiangyiwen wrote on Sat, Jul 14, 2018: >> When client has multiple threads that issue io requests all the >> time, and the server has a very good performance, it may cause >> cpu is running in the irq context for a long time because it can >> check virtqueue has buf in the *while* loop. >> >> So we should keep chan->lock in the whole loop. > > Hmm, this is generally bad practice to hold a spin lock for long. > In general, spin locks are meant to protect data, not code. > > I'd want some numbers to decide on this one, even if I think this > particular case is safe (e.g. this cannot dead-lock) >
Actually, the loop will not hold a spin lock for long, because other threads will not issue new requests in this case. In addition, virtio-blk or virtio-scsi also use this solution, I guess it may also encounter this problem before.
>> Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com> >> --- >> net/9p/trans_virtio.c | 8 +++----- >> 1 file changed, 3 insertions(+), 5 deletions(-) >> >> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c >> index 05006cb..9b0f5f2 100644 >> --- a/net/9p/trans_virtio.c >> +++ b/net/9p/trans_virtio.c >> @@ -148,20 +148,18 @@ static void req_done(struct virtqueue *vq) >> >> p9_debug(P9_DEBUG_TRANS, ": request done\n"); >> >> + spin_lock_irqsave(&chan->lock, flags); >> while (1) { >> - spin_lock_irqsave(&chan->lock, flags); >> req = virtqueue_get_buf(chan->vq, &len); >> - if (req == NULL) { >> - spin_unlock_irqrestore(&chan->lock, flags); >> + if (req == NULL) >> break; >> - } >> chan->ring_bufs_avail = 1; >> - spin_unlock_irqrestore(&chan->lock, flags); >> /* Wakeup if anyone waiting for VirtIO ring space. */ >> wake_up(chan->vc_wq); > > In particular, the wake up here echoes to wait events that will > immediately try to grab the lock, and will needlessly spin on it until > this thread is done. > If we do go this way I'd want setting chan->ring_bufs_avail to be done > just before unlocking and the wakeup to be done just after unlocking out > of the loop iff we processed at least one iteration here. >
I can move the wakeup operation after the unlocking. Like what I said above, I think this loop will not execute for long.
Thanks, Yiwen.
> That should also save you precious cpu cycles while under lock :) >
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