Re: Alternative to signals/sys_membarrier() in liburcu

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- Original Message -----
> From: "Linus Torvalds" <torvalds@linux-foundation.org>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
> Cc: "Michael Sullivan" <sully@msully.net>, lttng-dev@lists.lttng.org, "LKML" <linux-kernel@vger.kernel.org>, "Paul E.
> McKenney" <paulmck@linux.vnet.ibm.com>, "Peter Zijlstra" <peterz@infradead.org>, "Ingo Molnar" <mingo@kernel.org>,
> "Thomas Gleixner" <tglx@linutronix.de>, "Steven Rostedt" <rostedt@goodmis.org>
> Sent: Thursday, March 12, 2015 5:47:05 PM
> Subject: Re: Alternative to signals/sys_membarrier() in liburcu
> 
> On Thu, Mar 12, 2015 at 1:53 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
> >
> > 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 ?
> 
> Be *very* careful.
> 
> Just yesterday, in another thread (discussing the auto-numa TLB
> performance regression), we were discussing skipping the TLB
> invalidates entirely if the mprotect relaxes the protections.
> 
> Because if you *used* to be read-only, and them mprotect() something
> so that it is read-write, there really is no need to send a TLB
> invalidate, at least on x86. You can just change the page tables, and
> *if* any entries are stale in the TLB they'll take a microfault on
> access and then just reload the TLB.
> 
> So mprotect() to a more permissive mode is not necessarily serializing.

The idea here is to always mprotect() to a more restrictive mode,
which should trigger the TLB shootdown.

> 
> Also, you need to make sure that your page is actually in memory,
> because otherwise the kernel may end up seeing "oh, it's not even
> present", and never flush the TLB at all.
> 
> So now you need to mlock that page. Which can be problematic for non-root.

I'm aware the default amount of locked memory is usually quite low
(64kB here). So we'd need to handle cases where we run out of locked
memory. We could fallback to a slower userspace RCU scheme if this
occurs.

> 
> In other words, I'd be a bit leery about it. There may be other
> gotcha's about it.

Looking again at this old proposed patch (https://lkml.org/lkml/2010/4/18/15)
which adds a few memory barriers around updates to mm_cpumask
for sys_membarrier makes me wonder whether mprotect() may not skip
some CPU from the mask that would actually need to be taken care of
in very narrow race scenarios.

Thanks,

Mathieu


> 
>                       Linus
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com