linux-kernel - Re: Memory barrier needed with wake_up

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Date:   Mon, 5 Sep 2016 10:43:11 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Memory barrier needed with wake_up_process()?

On Sat, Sep 03, 2016 at 12:16:29AM +0200, Peter Zijlstra wrote:
> On Sat, Sep 03, 2016 at 12:14:13AM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 02, 2016 at 04:16:54PM -0400, Alan Stern wrote:
> > > 
> > > Actually, that's not entirely true (although presumably it works okay
> > > for most architectures).
> > 
> > Yeah, all load-store archs (with exception of PowerPC and ARM64 and
> > possibly MIPS) implement ACQUIRE with a general fence (after the ll/sc).
> > 
> > ( and MIPS doesn't use their fancy barriers in Linux )
> > 
> > PowerPC does the full fence for smp_mb__before_spinlock, which leaves
> > ARM64, I'm not sure its correct, but I'm way too tired to think about
> > that now.
> > 
> > The TSO archs imply full barriers with all atomic RmW ops and are
> > therefore also good.
> > 
> 
> Forgot to Cc Will. Will, does ARM64 need to make smp_mb__before_spinlock
> smp_mb() too?

Yes, probably. Just to confirm, the test is something like:


CPU0
----

Wx=1
smp_mb__before_spinlock()
LOCK(y)
Rz=0

CPU1
----

Wz=1
smp_mb()
Rx=0


and that should be forbidden?

Will