lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Wed, 6 Mar 2019 09:30:39 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: "H.J. Lu" <hjl.tools@...il.com>, libc-alpha <libc-alpha@...rceware.org>, Thomas Gleixner <tglx@...utronix.de>, linux-kernel <linux-kernel@...r.kernel.org>, linux-api <linux-api@...r.kernel.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Boqun Feng <boqun.feng@...il.com>, Andy Lutomirski <luto@...capital.net>, Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Russell King <linux@....linux.org.uk>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>, Josh Triplett <josh@...htriplett.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Michael Kerrisk <mtk.manpages@...il.com>, Joel Fernandes <joelaf@...gle.com>, Carlos O'Donell <carlos@...hat.com>, Florian Weimer <fweimer@...hat.com> Subject: Re: [PATCH for 5.1 0/3] Restartable Sequences updates for 5.1 On Tue, Mar 05, 2019 at 05:32:10PM -0500, Mathieu Desnoyers wrote: > >> * Adaptative mutex improvements > >> > >> I have done a prototype using rseq to implement an adaptative mutex which > >> can detect preemption using a rseq critical section. This ensures the > >> thread doesn't continue to busy-loop after it returns from preemption, and > >> calls sys_futex() instead. This is part of a user-space prototype branch [2], > >> and does not require any kernel change. > > > > I'm still not convinced that is actually the right way to go about > > things. The kernel heuristic is spin while the _owner_ runs, and we > > don't get preempted, obviously. > > > > And the only userspace spinning that makes sense is to cover the cost of > > the syscall. Now Obviously PTI wrecked everything, but before that > > syscalls were actually plenty fast and you didn't need many cmpxchg > > cycles to amortize the syscall itself -- which could then do kernel side > > adaptive spinning (when required). > > Indeed with PTI the system calls are back to their slow self. ;) > > You point about owner is interesting. Perhaps there is one tweak that I > should add in there. We could write the owner thread ID in the lock word. This is already required for PI (and I think robust) futexes. There have been proposals for FUTEX_LOCK and FUTEX_UNLOCK (!PI) primitives that require the same. Waiman had some patches; but I think all went under because 'important' stuff happened. > When trying to grab a lock, one of a few situations can happen: > > - It's unlocked, so we grab it by storing our thread ID, > - It's locked, and we can fetch the CPU number of the thread owning it > if we can access its (struct rseq *)->cpu_id through a lookup using its > thread ID, We can then check whether it's the same CPU we are running on. That might just work with threads (private futexes; which are the majority these these I think), but will obviously not work with regular (shared) futexes. > - If so, we _know_ we should let the owner run, so we call futex right away, > no spinning. We can even boost it for priority inheritance mutexes, > - If it's owned by a thread which was last running on a different CPU, > then it may make sense to actively try to grab the lock by spinning > up to a certain number of loops (which can be either fixed or adaptative). > After that limit, call futex. If preempted while looping, call futex. > > Do you see this as an improvement over what exists today, or am I > on the wrong track ? That's probably better than what they have today. Last time I looked at libc pthread I got really sad -- arguably that was a long time ago, and some of that stuff is because POSIX, but still. Some day we should redesign all that.. futex2 etc.
Powered by blists - more mailing lists