Re: Alternative to signals/sys_membarrier() in liburcu

From: One Thousand Gnomes
Date: Thu Mar 12 2015 - 20:01:01 EST

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

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