linux-kernel - Re: [PATCH 0/3] SCSI: Fix hard lockup in scsi_remove

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Date:	Wed, 14 Oct 2015 16:22:10 -0400
From:	Ewan Milne <emilne@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Johannes Thumshirn <jthumshirn@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Hannes Reinecke <hare@...e.de>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] SCSI: Fix hard lockup in scsi_remove_target()

On Wed, 2015-10-14 at 07:30 -0700, James Bottomley wrote:
> On Wed, 2015-10-14 at 15:50 +0200, Johannes Thumshirn wrote:
> > Removing a SCSI target via scsi_remove_target() suspected to be racy. When a
> > sibling get's removed from the list it can occassionly happen that one CPU is
> > stuck endlessly looping around this code block
> > 
> > list_for_each_entry(starget, &shost->__targets, siblings) {
> >         if (starget->state == STARGET_DEL)
> >                 continue;
> 
> How long is the __targets list?  It seems a bit unlikely that this is
> the exact cause, because for a short list all in STARGET_DEL that loop
> should exit very quickly.  Where in the code does scsi_remove_target
> +0x68/0x240 actually point to?
> 
> Is it not a bit more likely that we're following a removed list element?
> Since that points back to itself, the list_for_each_entry() would then
> circulate forever.  If that's the case the simple fix would be to use
> the safe version of the list traversal macro.
> 
> James

For what it's worth, I've seen a dump where this was exactly the case.
starget was in STARGET_DEL state, starget->siblings pointed to itself,
kref was 0, reap_ref was 0 (this was a while back).

The problem was not able to be reproduced at the time.

-Ewan


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