linux-kernel - Re: Memory barrier needed with wake_up

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Message-ID: <20160902221629.GK10168@twins.programming.kicks-ass.net>
Date:   Sat, 3 Sep 2016 00:16:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "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>,
        Will Deacon <will.deacon@....com>
Subject: Re: Memory barrier needed with wake_up_process()?

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?