Re: [V9fs-developer] [PATCH] net/9p: Fix a deadlock case in the virtio transport

[Dominique Martinet <asmadeus@codewreck.org>]

jiangyiwen wrote on Sat, Jul 14, 2018:
> 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.

Fair enough. If you do have some numbers to give though (throughput
and/or iops before/after) I'd still be really curious.

> >>  		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.

Please do, you listed virtio_blk as doing this and they have the same
kind of pattern with a req_done bool and only restarting stopped queues
if they processed something

-- 
Dominique