linux-kernel - Re: smp_mb__after_spinlock requirement too strong?

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Date:   Mon, 12 Mar 2018 09:56:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     焦晓冬 <milestonejxd@...il.com>,
        linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>, will.deacon@....com,
        torvalds@...ux-foundation.org, npiggin@...il.com, mingo@...nel.org,
        mpe@...erman.id.au, oleg@...hat.com, benh@...nel.crashing.org,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: smp_mb__after_spinlock requirement too strong?

On Mon, Mar 12, 2018 at 04:56:00PM +0800, Boqun Feng wrote:
> So I think the purpose of smp_mb__after_spinlock() is to provide RCsc
> locks, it's just the comments before that may be misleading. We want
> RCsc locks in schedule code because we want writes in different critical
> section are ordered even outside the critical sections, for case like:
> 
> 	CPU 0		CPU 1		CPU 2
> 
> 	{A =0 , B = 0}
> 	lock(rq0);
> 	write A=1;
> 	unlock(rq0);
> 
> 			lock(rq0);
> 			read A=1;
> 			write B=2;
> 			unlock(rq0);
> 
> 					read B=2;
> 					smp_rmb();
> 					read A=1;
> 
> I think we need to fix the comments rather than loose the requirement.
> Peter?

Yes, ISTR people relying on schedule() being RCsc, and I just picked a
bad exmaple.