linux-kernel - Re: [RFC PATCH for 4.18 00/16] Restartable Sequences

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Message-ID: <20180728141314.GA25264@amd>
Date:   Sat, 28 Jul 2018 16:13:14 +0200
From:   Pavel Machek <pavel@....cz>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <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>, 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>
Subject: Re: [RFC PATCH for 4.18 00/16] Restartable Sequences

Hi!

> Documentation-wise, I have posted a rseq man page rfc here:
> 
> https://lkml.kernel.org/r/20180616195803.29877-1-mathieu.desnoyers@efficios.com
> 
> comments are welcome!

Thanks for pointer.

+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).

So the threads are protected against sigkill when running the
restartable sequence?

+Restartable sequences must not perform system calls. Doing so may
result
+in termination of the process by a segmentation fault.
+

"may result"? It would be nice to always catch that.

+Optimistic cache of the CPU number on which the current thread is
+running. Its value is guaranteed to always be a possible CPU number,
+even when rseq is not initialized. The value it contains should
always
+be confirmed by reading the cpu_id field.

I'm not sure what "optimistic cache" is...

+Flags indicating the restart behavior for the current thread. This is
+mainly used for debugging purposes. Can be either:
+.IP \[bu]
+RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
+.IP \[bu]
+RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
+.IP \[bu]

Flags tell me there may be more then one, but "can be either" tells me
just one flag is allowed.

+.B Structure alignment
+This structure is aligned on multiples of 32 bytes.
+.TP
+.B Structure size
+This structure has a fixed size of 32 bytes.
+.B Structure alignment
+This structure is aligned on multiples of 32 bytes.
+.TP
+.B Structure size
+This structure has a fixed size of 32 bytes.

I believe we normally say "is aligned on 32-bytes boundary". (Will not
this need to be bigger on machines with bigger cache sizes?)

above it says:

+.B Structure size
+This structure is extensible. Its size is passed as parameter to the
+rseq system call.

I'm reading source, so maybe it refers to different structure.

Thanks,
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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