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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20170419232457.GD3956@linux.vnet.ibm.com>
Date:   Wed, 19 Apr 2017 16:24:57 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 07:59:07PM +0200, Peter Zijlstra wrote:
> 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).

Good points, and my proposed patch includes verbiage urging the use
of something else to get the job done.  Does that work?

							Thanx, Paul