----- Original Message ----- > From: "One Thousand Gnomes" <[email protected]> > To: "Mathieu Desnoyers" <[email protected]> > Cc: "Michael Sullivan" <[email protected]>, "Peter Zijlstra" > <[email protected]>, "LKML" > <[email protected]>, "Steven Rostedt" <[email protected]>, > [email protected], "Thomas Gleixner" > <[email protected]>, "Paul E. McKenney" <[email protected]>, "Linus > Torvalds" > <[email protected]>, "Ingo Molnar" <[email protected]> > Sent: Thursday, March 12, 2015 7:59:38 PM > Subject: Re: Alternative to signals/sys_membarrier() in liburcu > > On Thu, 12 Mar 2015 20:56:00 +0000 (UTC) > Mathieu Desnoyers <[email protected]> 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.
I'd claim that removing the IPIs breaks userspace, of course. :-P If we start relying on mprotect() implying memory barriers issued on all CPUs associated with the memory mapping in core user-space libraries, then whenever those shiny new CPUs show up, we might be stuck with the IPIs, otherwise we could claim that removing them breaks userspace. I would really hate to tie in an assumption like that on mprotect, because that would really be painting ourselves in a corner. > > Also doesn't sun4d have hardware shootdown for 16 processors or less ? That's possible. I'm no sun expert though. > > 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) I'd really prefer a well-documented system call for that purpose too. Thanks, Mathieu > > Alan > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
