linux-kernel - Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work

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Date:   Thu, 13 Apr 2017 19:59:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com, dvyukov@...gle.com,
        will.deacon@....com
Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to
 sync_exp_work_done()

On Thu, Apr 13, 2017 at 07:51:36PM +0200, Peter Zijlstra wrote:

> > I suppose that one alternative is the new variant of kerneldoc, though
> > very few of these functions have comment headers, let alone kerneldoc
> > headers.  Which reminds me, the question of spin_unlock_wait() and
> > spin_is_locked() semantics came up a bit ago.  Here is what I believe
> > to be the case.  Does this match others' expectations?
> > 
> > o	spin_unlock_wait() semantics:
> > 
> > 	1.	Any access in any critical section prior to the
> > 		spin_unlock_wait() is visible to all code following
> > 		(in program order) the spin_unlock_wait().
> > 
> > 	2.	Any access prior (in program order) to the
> > 		spin_unlock_wait() is visible to any critical
> > 		section following the spin_unlock_wait().
> > 
> > o	spin_is_locked() semantics: Half of spin_unlock_wait(),
> > 	but only if it returns false:
> > 
> > 	1.	Any access in any critical section prior to the
> > 		spin_unlock_wait() is visible to all code following
> > 		(in program order) the spin_unlock_wait().
> 
> Urgh.. yes those are pain. The best advise is to not use them.
> 
>   055ce0fd1b86 ("locking/qspinlock: Add comments")

The big problem with spin_unlock_wait(), aside from the icky barrier
semantics, is that it tends to end up prone to starvation. So where
spin_lock()+spin_unlock() have guaranteed fwd progress if the lock is
fair (ticket,queued,etc..) spin_unlock_wait() must often lack that
guarantee.

Equally, spin_unlock_wait() was intended to be 'cheap' and be a
read-only loop, but in order to satisfy the barrier requirements, it
ends up doing stores anyway (see for example the arm64 and ppc
implementations).