linux-kernel - Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED

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Date:	Sat, 22 Feb 2014 10:52:18 -0800
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Tejun Heo <tj@...nel.org>, laijs@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux1394-devel@...ts.sourceforge.net,
	Chris Boot <bootc@...tc.net>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

On Sat, 2014-02-22 at 13:48 -0500, Peter Hurley wrote:
> On 02/22/2014 01:43 PM, James Bottomley wrote:
> >
> > On Fri, 2014-02-21 at 18:01 -0500, Peter Hurley wrote:
> >> On 02/21/2014 11:57 AM, Tejun Heo wrote:
> >>> Yo,
> >>>
> >>> On Fri, Feb 21, 2014 at 11:53:46AM -0500, Peter Hurley wrote:
> >>>> Ok, I can do that. But AFAIK it'll have to be an smp_rmb(); there is
> >>>> no mb__after unlock.
> >>>
> >>> We do have smp_mb__after_unlock_lock().
> >>>
> >>>> [ After thinking about it some, I don't think preventing speculative
> >>>>     writes before clearing PENDING if useful or necessary, so that's
> >>>>     why I'm suggesting only the rmb. ]
> >>>
> >>> But smp_mb__after_unlock_lock() would be cheaper on most popular
> >>> archs, I think.
> >>
> >> smp_mb__after_unlock_lock() is only for ordering memory operations
> >> between two spin-locked sections on either the same lock or by
> >> the same task/cpu. Like:
> >>
> >>      i = 1
> >>      spin_unlock(lock1)
> >>      spin_lock(lock2)
> >>      smp_mb__after_unlock_lock()
> >>      j = 1
> >>
> >> This guarantees that the store to j happens after the store to i.
> >> Without it, a cpu can
> >>
> >>      spin_lock(lock2)
> >>      j = 1
> >>      i = 1
> >>      spin_unlock(lock1)
> >
> > No the CPU cannot.  If the CPU were allowed to reorder locking
> > sequences, we'd get speculation induced ABBA deadlocks.  The rules are
> > quite simple: loads and stores cannot speculate out of critical
> > sections.
> 
> If you look carefully, you'll notice that the stores have not been
> moved from their respective critical sections; simply that the two
> critical sections overlap because they use different locks.

You didn't look carefully enough at what I wrote.  You may not reorder
critical sections so they overlap regardless of whether the locks are
independent or not.  This is because we'd get ABBA deadlocks due to
speculation (A represents lock1 and B lock 2 in your example).

James


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