linux-kernel - Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount

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Message-ID: <20150617165802.GF19282@twins.programming.kicks-ass.net>
Date:	Wed, 17 Jun 2015 18:58:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

On Wed, Jun 17, 2015 at 08:42:45AM -0700, Paul E. McKenney wrote:
> > I would very much prefer a compiler switch that instructs the compiler
> > to not do bloody stupid things like this instead of marking every other
> > load/store in the kernel with volatile.
> 
> I would of course be good with such a compiler switch, though my earlier
> attempts to negotiate one were unsuccessful.  But I don't believe that we
> discussed a switch to specifically prohibit only use of to-be-stored-into
> variables as temporary scratch space.  The trick is finding restrictions
> that are useful, but that don't imply -O0.

I would request on that disables all the 'stores from thin air'
'optimizations'. IOW assume everything is shared memory and concurrent
unless you can prove its not so. For example a local stack variable that
does not escape scope.
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