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

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Date:	Wed, 14 Oct 2015 08:45:56 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Johannes Thumshirn <jthumshirn@...e.de>
Cc:	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 16:39 +0200, Johannes Thumshirn wrote:
> 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.
> 
> Yes it is traversing a removed element and yes the patches 2/3 and 3/3
> are introducing the safe version of list_for_each_entry(), but they
> also decouple the search for elements to be removed from the actual
> removal. This is what my initial proposal did as well. Christoph wanted
> me to decouple the whole process from the host_lock though and this is
> what this patches do as well now.

OK, so I really need you to separate the problems.  Fixing the bug
you're reporting does not require a complete rework of the locking
infrastructure; it just requires replacing the traversal macro with the
safe version, can you verify that and it can go into fixes?

Then we can discuss the merits of doing a locking rework in this area
separately from the idea that it's some sort of bug fix.

James


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