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

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Date:	Wed, 11 Nov 2015 17:39:40 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	corbet@....net, mhocko@...nel.org, dhowells@...hat.com,
	torvalds@...ux-foundation.org, will.deacon@....com
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Hi Oleg,

On Tue, Nov 03, 2015 at 06:59:58PM +0100, Oleg Nesterov wrote:
[snip]
> 
> Unfortunately this doesn't look exactly right...
> 
> spin_unlock_wait() is not equal to "while (locked) relax", the latter
> is live-lockable or at least sub-optimal: we do not really need to spin

Just be curious, should spin_unlock_wait() semantically be an ACQUIRE?
Because spin_unlock_wait() is used for us to wait for a certain lock to
RELEASE so that we can do something *after* we observe the RELEASE.
Considering the follow example:

	CPU 0 				CPU 1
	============================	===========================
	{ X = 0 }
					WRITE_ONCE(X, 1);
					spin_unlock(&lock);
	spin_unlock_wait(&lock)
	r1 = READ_ONCE(X);

If spin_unlock_wait() is not an ACQUIRE, r1 can be 0 in this case,
right? Am I missing something subtle here? Or spin_unlock_wait() itself
doesn't have the ACQUIRE semantics, but it should always come with a
smp_mb() *following* it to achieve the ACQUIRE semantics? However in
do_exit(), an smp_mb() is preceding raw_spin_unlock_wait() rather than
following, which makes me confused... could you explain that? Thank you
;-)

Regards,
Boqun

> until we observe !spin_is_locked(), we only need to synchronize with the
> current owner of this lock. Once it drops the lock we can proceed, we
> do not care if another thread takes the same lock right after that.
> 
> Oleg.
> 

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