linux-kernel - Re: Question on smp_mb__before

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Date:   Mon, 5 Sep 2016 06:57:37 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: Question on smp_mb__before_spinlock

On Mon, Sep 05, 2016 at 01:34:35PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 05, 2016 at 03:37:14AM -0700, Paul E. McKenney wrote:
> > On Mon, Sep 05, 2016 at 11:37:53AM +0200, Peter Zijlstra wrote:
> > > Hi all,
> > > 
> > > So recently I've had two separate issues that touched upon
> > > smp_mb__before_spinlock().
> > > 
> > > 
> > > Since its inception, our understanding of ACQUIRE, esp. as applied to
> > > spinlocks, has changed somewhat. Also, I wonder if, with a simple
> > > change, we cannot make it provide more.
> > > 
> > > The problem with the comment is that the STORE done by spin_lock isn't
> > > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
> > > it and cross with any prior STORE, rendering the default WMB
> > > insufficient (pointed out by Alan).
> > > 
> > > Now, this is only really a problem on PowerPC and ARM64, the former of
> > > which already defined smp_mb__before_spinlock() as a smp_mb(), the
> > > latter does not, Will?
> > > 
> > > The second issue I wondered about is spinlock transitivity. All except
> > > powerpc have RCsc locks, and since Power already does a full mb, would
> > > it not make sense to put it _after_ the spin_lock(), which would provide
> > > the same guarantee, but also upgrades the section to RCsc.
> > > 
> > > That would make all schedule() calls fully transitive against one
> > > another.
> > > 
> > > 
> > > That is, would something like the below make sense?
> > 
> > Looks to me like you have reinvented smp_mb__after_unlock_lock()...
> 
> Will said the same, but that one doesn't in fact do the first bit, as
> ARM64 also needs a full barrier for that, while it doesn't need that to
> upgrade to RCsc.

Fair enough!

							Thanx, Paul