linux-kernel - Re: [Xen-devel] [PATCH 1/3] xen/blkback: fix disconnect while I/Os in flight

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Date:   Tue, 16 May 2017 11:28:14 +0200
From:   Dietmar Hahn <dietmar.hahn@...fujitsu.com>
To:     xen-devel@...ts.xen.org
Cc:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, roger.pau@...rix.com,
        netwiz@....id.au, stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 1/3] xen/blkback: fix disconnect while I/Os in flight

Am Dienstag, 16. Mai 2017, 08:23:18 schrieb Juergen Gross:
> Today disconnecting xen-blkback is broken in case there are still
> I/Os in flight: xen_blkif_disconnect() will bail out early without
> releasing all resources in the hope it will be called again when
> the last request has terminated. This, however, won't happen as
> xen_blkif_free() won't be called on termination of the last running
> request: xen_blkif_put() won't decrement the blkif refcnt to 0 as
> xen_blkif_disconnect() didn't finish before thus some xen_blkif_put()
> calls in xen_blkif_disconnect() didn't happen.
> 
> To solve this deadlock xen_blkif_disconnect() and
> xen_blkif_alloc_rings() shouldn't use xen_blkif_put() and
> xen_blkif_get() but use some other way to do their accounting of
> resources.
> 
> This at once fixes another error in xen_blkif_disconnect(): when it
> returned early with -EBUSY for another ring than 0 it would call
> xen_blkif_put() again for already handled rings on a subsequent call.
> This will lead to inconsistencies in the refcnt handling.
> 
> Cc: stable@...r.kernel.org
> Reported-by: Glenn Enright <glenn@...uhosting.com>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> Tested-by: Steven Haigh <netwiz@....id.au>
> ---
>  drivers/block/xen-blkback/common.h | 1 +
>  drivers/block/xen-blkback/xenbus.c | 7 +++++--
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index dea61f6ab8cb..953f38802333 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -281,6 +281,7 @@ struct xen_blkif_ring {
>  
>  	wait_queue_head_t	wq;
>  	atomic_t		inflight;
> +	int			active;

Maybe better using bool?

Dietmar.

>  	/* One thread per blkif ring. */
>  	struct task_struct	*xenblkd;
>  	unsigned int		waiting_reqs;
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 1f3dfaa54d87..e68df9de8858 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -159,7 +159,7 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
>  		init_waitqueue_head(&ring->shutdown_wq);
>  		ring->blkif = blkif;
>  		ring->st_print = jiffies;
> -		xen_blkif_get(blkif);
> +		ring->active = 1;
>  	}
>  
>  	return 0;
> @@ -249,6 +249,9 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
>  		struct xen_blkif_ring *ring = &blkif->rings[r];
>  		unsigned int i = 0;
>  
> +		if (!ring->active)
> +			continue;
> +
>  		if (ring->xenblkd) {
>  			kthread_stop(ring->xenblkd);
>  			wake_up(&ring->shutdown_wq);
> @@ -296,7 +299,7 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
>  		BUG_ON(ring->free_pages_num != 0);
>  		BUG_ON(ring->persistent_gnt_c != 0);
>  		WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));
> -		xen_blkif_put(blkif);
> +		ring->active = 0;
>  	}
>  	blkif->nr_ring_pages = 0;
>  	/*
> 

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