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

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Date:	Wed, 14 Oct 2015 17:36:28 +0000
From:	Johannes Thumshirn <jthumshirn@...e.de>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
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()

Zitat von James Bottomley <James.Bottomley@...senPartnership.com>:

> 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?
>

Yes. I can sent a patch for it tomorrow.



> 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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