Re: [PATCH 0/3] SCSI: Fix hard lockup in scsi_remove_target()

[Johannes Thumshirn]

Zitat von James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>:

> 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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