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" <sully@xxxxxxxxxx> > To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx> > Cc: lttng-dev@xxxxxxxxxxxxxxx > 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 < mathieu.desnoyers@xxxxxxxxxxxx > 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 > > _______________________________________________ > lttng-dev mailing list > lttng-dev@xxxxxxxxxxxxxxx > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
