linux-kernel - Re: Alternative to signals/sys

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Date:	Thu, 12 Mar 2015 23:59:38 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Michael Sullivan <sully@...lly.net>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	lttng-dev@...ts.lttng.org, Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: Alternative to signals/sys_membarrier() in liburcu

On Thu, 12 Mar 2015 20:56:00 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> (sorry for re-send, my mail client tricked me into posting HTML
> to lkml)
> 
> Hi, 
> 
> Michael Sullivan proposed a clever hack abusing mprotect() to 
> perform the same effect as sys_membarrier() I submitted a few 
> years ago ( https://lkml.org/lkml/2010/4/18/15 ). 
> 
> At that time, the sys_membarrier implementation was deemed 
> technically sound, but there were not enough users of the system call 
> to justify its inclusion. 
> 
> So far, the number of users of liburcu has increased, but liburcu 
> still appears to be the only direct user of sys_membarrier. On this 
> front, we could argue that many other system calls have only 
> one user: glibc. In that respect, liburcu is quite similar to glibc. 
> 
> So the question as it stands appears to be: would you be comfortable 
> having users abuse mprotect(), relying on its side-effect of issuing 
> a smp_mb() on each targeted CPU for the TLB shootdown, as 
> an effective implementation of process-wide memory barrier ? 

What are you going to do if some future ARM or x86 CPU update with
hardware TLB shootdown appears ? All your code will start to fail on new
kernels using that property, and in nasty insidious ways.

Also doesn't sun4d have hardware shootdown for 16 processors or less ?

I would have thought a membarrier was a lot safer and it can be made to
do whatever horrible things are needed on different processors (indeed it
could even be a pure libc hotpath if some future cpu grows this ability)

Alan
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