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Message-ID: <20150617154245.GB3913@linux.vnet.ibm.com>
Date: Wed, 17 Jun 2015 08:42:45 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
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 05:11:09PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 17, 2015 at 07:57:12AM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 17, 2015 at 02:29:24PM +0200, Peter Zijlstra wrote:
> > > I did leave off the READ/WRITE ONCE stuff, because I could not come up
> > > with a scenario where it makes a difference -- I appreciate paranoia,
> > > but I also think we should not overdo the thing.
> >
> > I can only conclude that you have not read this document:
> >
> > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html
>
> This would be correct.
>
> > Specifically, please keep in mind that unless you mark either the variable
> > or the memory access, the compiler is within its rights to assume that
> > there are no concurrent accesses to that variable. For but one example,
> > if you do a normal store to a given variable, then the compiler is
> > within its rights to use that variable as temporary storage prior to
> > that store. And yes, you can reasonably argue that no sane compiler
> > would store something else to s->sequence given that it could free up
> > a register by storing the incremented value, but the fact remains that
> > you have given it permission to do so if it wants.
>
> Argh *grmbl*, that's bloody insane!
You expected me to argue with that statement? ;-)
> So I get the re-loading, I get the tearing, but this random intermittent
> values (somewhat related to stores out of thin air) is completely
> bonkers.
>
> 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.
Any GCC or LLVM folks on the list?
> Note that if GCC were to actually do something like this, the kernel
> would already be broken, because I'm very sure we did not consider/audit
> it for this.
An accident waiting to happen, given that both GCC and the Linux kernel
are moving targets. :-/
Thanx, Paul
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