> Date: Wed, 24 Apr 2024 22:25:01 +0200 (CEST)
> From: Stefan Fritsch <sf@sfritsch.de>
> 
> Hi,
> 
> it seems I went down the wrong rabbit hole with my first diff. The actual 
> issue is that signals are not handled correctly while sleeping. After a 
> signal, there is a race condition where sc_ctrl_inuse is first set to FREE 
> and then the interrupt handler sets it to DONE, causing a hang in the next 
> vio_wait_ctrl() call. This was probably not noticed before NET_LOCK 
> existed, because an ifconfig down/up fixed it. However since the NET_LOCK 
> now serializes the relevant ioctl paths, a parallel ifconfig down now 
> hangs too, waiting for the NET_LOCK. The attached patch does three things:
> 
> * Revert the NET_LOCK unlocking work-around.
> 
> * Remove PCATCH from the sleep calls so that we always wait for the control
>   requests to complete, avoiding the race with vio_ctrleof().

What guarantee is there that those control requests complete?

> * Avoid a crash if there is outgoing traffic while doing ifconfig down. 
>   The proper fix for this would be intr_barrier() but I think that would 
>   need more refactoring to avoid problems with the NET_LOCK.
> 
> With this I have seen neither crashes nor deadlocks. I would like to 
> commit this and do further refactoring in separate steps.
> 
> With the NET_LOCK, the code to handle control requests concurrent to a 
> device reset cannot be hit anymore. Also it cannot happen that we have to 
> wait for a free slot in the control queue. If we assume that there will 
> always be some lock serializing the ioctl path (maybe with some 
> per-interface lock to replace NET_LOCK), this code could be removed. Or 
> vio could introduce its own cfg_lock like ixl(4) or cad(4).
> 
> Comments, testers (especially with v6 autoconf), oks would be very 
> welcome.
> 
> Cheers,
> Stefan
> 
> diff --git a/sys/dev/pv/if_vio.c b/sys/dev/pv/if_vio.c
> index 9ad8e36e346..7a49d1a8fdf 100644
> --- a/sys/dev/pv/if_vio.c
> +++ b/sys/dev/pv/if_vio.c
> @@ -1232,13 +1232,17 @@ vio_txeof(struct virtqueue *vq)
>  
>  	while (virtio_dequeue(vsc, vq, &slot, &len) == 0) {
>  		struct virtio_net_hdr *hdr = &sc->sc_tx_hdrs[slot];
> +		m = sc->sc_tx_mbufs[slot];
> +		if (m == NULL) {
> +			// interface is down by now

Don't use C++-style comments.

> +			continue;
> +		}
>  		r++;
>  		VIO_DMAMEM_SYNC(vsc, sc, hdr, sc->sc_hdr_size,
>  		    BUS_DMASYNC_POSTWRITE);
>  		bus_dmamap_sync(vsc->sc_dmat, sc->sc_tx_dmamaps[slot], 0,
>  		    sc->sc_tx_dmamaps[slot]->dm_mapsize,
>  		    BUS_DMASYNC_POSTWRITE);
> -		m = sc->sc_tx_mbufs[slot];
>  		bus_dmamap_unload(vsc->sc_dmat, sc->sc_tx_dmamaps[slot]);
>  		sc->sc_tx_mbufs[slot] = NULL;
>  		virtio_dequeue_commit(vq, slot);
> @@ -1363,32 +1367,13 @@ out:
>  	return r;
>  }
>  
> -/*
> - * XXXSMP As long as some per-ifp ioctl(2)s are executed with the
> - * NET_LOCK() deadlocks are possible.  So release it here.
> - */
> -static inline int
> -vio_sleep(struct vio_softc *sc, const char *wmesg)
> -{
> -	int status = rw_status(&netlock);
> -
> -	if (status != RW_WRITE && status != RW_READ)
> -		return tsleep_nsec(&sc->sc_ctrl_inuse, PRIBIO|PCATCH, wmesg,
> -		    INFSLP);
> -
> -	return rwsleep_nsec(&sc->sc_ctrl_inuse, &netlock, PRIBIO|PCATCH, wmesg,
> -	    INFSLP);
> -}
> -
>  int
>  vio_wait_ctrl(struct vio_softc *sc)
>  {
>  	int r = 0;
>  
>  	while (sc->sc_ctrl_inuse != FREE) {
> -		r = vio_sleep(sc, "viowait");
> -		if (r == EINTR)
> -			return r;
> +		r = tsleep_nsec(&sc->sc_ctrl_inuse, PRIBIO, "viowait", INFSLP);
>  	}
>  	sc->sc_ctrl_inuse = INUSE;
>  
> @@ -1402,12 +1387,10 @@ vio_wait_ctrl_done(struct vio_softc *sc)
>  
>  	while (sc->sc_ctrl_inuse != DONE && sc->sc_ctrl_inuse != RESET) {
>  		if (sc->sc_ctrl_inuse == RESET) {
> -			r = 1;
> +			r = ENXIO;
>  			break;
>  		}
> -		r = vio_sleep(sc, "viodone");
> -		if (r == EINTR)
> -			break;
> +		r = tsleep_nsec(&sc->sc_ctrl_inuse, PRIBIO, "viodone", INFSLP);
>  	}
>  	return r;
>  }
> 
>