On Thu, Mar 12, 2015 at 08:56:00PM +0000, Mathieu Desnoyers 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 ?
>
> Thoughts ?
Are there any architectures left that use hardware-assisted global
TLB invalidation? On such an architecture, you might not get a memory
barrier except on the CPU executing the mprotect() or munmap().
(Here is hoping that no one does -- it is a cute abuse^Whack otherwise!)
Thanx, Paul
> Thanks!
>
> Mathieu
>
>
>
>
>
> From: "Michael Sullivan" <[email protected]>
> To: "Mathieu Desnoyers" <[email protected]>
> Cc: [email protected]
> Sent: Thursday, March 12, 2015 12:04:07 PM
> Subject: Re: [lttng-dev] Alternative to signals/sys_membarrier() in liburcu
>
> On Thu, Mar 12, 2015 at 10:57 AM, Mathieu Desnoyers <
> [email protected] > wrote:
>
>
>
>
> Even though it depends on internal behavior not currently specified by
> mprotect,
> I'd very much like to see the prototype you have,
>
>
> I ended up posting my code at
> https://github.com/msullivan/userspace-rcu/tree/msync-barrier .
> The interesting patch is
> https://github.com/msullivan/userspace-rcu/commit/04656b468d418efbc5d934ab07954eb8395a7ab0
> .
>
> Quick blog post I wrote about it at
> http://www.msully.net/blog/2015/02/24/forcing-memory-barriers-on-other-cpus-with-mprotect2/
> .
> (I talked briefly about sys_membarrier in the post as best as I could piece
> together from LKML; if my comment on it is inaccurate I can edit the post.)
>
> -Michael Sullivan
>
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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