linux-kernel - Re: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index

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Message-ID: <20110609091433.GB11773@htj.dyndns.org>
Date:	Thu, 9 Jun 2011 11:14:33 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mark Wu <dwu@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH 1/1] [virt] virtio-blk: Use ida to allocate disk index

Hello,

On Thu, Jun 09, 2011 at 08:51:05AM +0930, Rusty Russell wrote:
> On Wed, 08 Jun 2011 09:08:29 -0400, Mark Wu <dwu@...hat.com> wrote:
> > Hi Rusty,
> > Yes, I can't figure out an instance of disk probing in parallel either, but as
> > per the following commit, I think we still need use lock for safety. What's your opinion?
> > 
> > commit 4034cc68157bfa0b6622efe368488d3d3e20f4e6
> > Author: Tejun Heo <tj@...nel.org>
> > Date:   Sat Feb 21 11:04:45 2009 +0900
> > 
> >     [SCSI] sd: revive sd_index_lock
> > 
> >     Commit f27bac2761cab5a2e212dea602d22457a9aa6943 which converted sd to
> >     use ida instead of idr incorrectly removed sd_index_lock around id
> >     allocation and free.  idr/ida do have internal locks but they protect
> >     their free object lists not the allocation itself.  The caller is
> >     responsible for that.  This missing synchronization led to the same id
> >     being assigned to multiple devices leading to oops.
> 
> I'm confused.  Tejun, Greg, anyone can probes happen in parallel?
> 
> If so, I'll have to review all my drivers.

Unless async is explicitly used, probe happens sequentially.  IOW, if
there's no async_schedule() call, things won't happen in parallel.
That said, I think it wouldn't be such a bad idea to protect ida with
spinlock regardless unless the probe code explicitly requires
serialization.

Thanks.

-- 
tejun
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