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Message-Id: <20180402152736.GL3948@linux.vnet.ibm.com> Date: Mon, 2 Apr 2018 08:27:36 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Christopher Lameter <cl@...ux.com> Cc: Alan Cox <gnomes@...rguk.ukuu.org.uk>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, Boqun Feng <boqun.feng@...il.com>, Andy Lutomirski <luto@...capital.net>, Dave Watson <davejwatson@...com>, linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Russell King <linux@....linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>, Andi Kleen <andi@...stfloor.org>, Ben Maurer <bmaurer@...com>, Steven 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>, Alexander Viro <viro@...iv.linux.org.uk> Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) On Mon, Apr 02, 2018 at 10:03:58AM -0500, Christopher Lameter wrote: > On Sun, 1 Apr 2018, Alan Cox wrote: > > > > Restartable sequences are atomic with respect to preemption > > > (making it atomic with respect to other threads running on the > > > same CPU), as well as signal delivery (user-space execution > > > contexts nested over the same thread). > > > > CPU generally means 'big lump with legs on it'. You are not atomic to the > > same CPU, because that CPU may have 30+ cores with 8 threads per core. > > > > It could do with some better terminology (hardware thread, CPU context ?) > > Well we call it a "CPU" in the scheduler context I think. We could use > better terminology throughout the kernel tools and source. Agreed, it has been "CPU" for "single hardware thread" for a very long time. People tend to use "core" for "group of hardware threads" and "socket" for "big lump with legs on it". > Hardware Execution Context? Should be even more fun when non-CPU hardware execution contexts show up in force within each core. ;-) But the terminology in place for non-CPU hardware execution contexts should be able to survive that event. > > > In a typical usage scenario, the thread registering the rseq > > > structure will be performing loads and stores from/to that > > > structure. It is however also allowed to read that structure > > > from other threads. The rseq field updates performed by the > > > kernel provide relaxed atomicity semantics, which guarantee > > > that other threads performing relaxed atomic reads of the cpu > > > number cache will always observe a consistent value. > > > > So what happens to your API if the kernel atomics get improved ? You are > > effectively exporting rseq behaviour from private to public. > > There is already a pretty complex coherency model guiding kernel atomics. > Improvements/changes to that are difficult and the effect will ripple > throughout the kernel. So I would suggest that these areas of the kernel > are pretty "petrified" (or written in stone). I suspect that there are much more pressing areas of confusion in any case! Thanx, Paul
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