linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Message-ID: <20151116160445.GK11639@twins.programming.kicks-ass.net>
Date:	Mon, 16 Nov 2015 17:04:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Boqun Feng <boqun.feng@...il.com>, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Mon, Nov 16, 2015 at 04:56:58PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 12, 2015 at 10:21:39AM -0800, Linus Torvalds wrote:
> > Now, the point of spin_unlock_wait() (and "spin_is_locked()") should
> > generally be that you have some external ordering guarantee that
> > guarantees that the lock has been taken. For example, for the IPC
> > semaphores, we do either one of:
> > 
> >  (a) get large lock, then - once you hold that lock - wait for each small lock
> > 
> > or
> > 
> >  (b) get small lock, then - once you hold that lock - check that the
> > largo lock is unlocked
> > 
> > and that's the case we should really worry about.  The other uses of
> > spin_unlock_wait() should have similar "I have other reasons to know
> > I've seen that the lock was taken, or will never be taken after this
> > because XYZ".
> 
> I don't think this is true for the usage in do_exit(), we have no
> knowledge on if pi_lock is taken or not. We just want to make sure that
> _if_ it were taken, we wait until it is released.

And unless PPC would move to using RCsc locks with a SYNC in
spin_lock(), I don't think it makes sense to add
smp_mb__after_unlock_lock() to all tsk->pi_lock instances to fix this.
As that is far more expensive than flipping the exit path to do
spin_lock()+spin_unlock().
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