linux-kernel - Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

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Message-ID: <374861479.8581.1470957990793.JavaMail.zimbra@efficios.com>
Date:	Thu, 11 Aug 2016 23:26:30 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Dave Watson <davejwatson@...com>
Cc:	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>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson@...com wrote:

>>> +static inline __attribute__((always_inline))
>>> +bool rseq_finish(struct rseq_lock *rlock,
>>> + intptr_t *p, intptr_t to_write,
>>> + struct rseq_state start_value)
> 
>>> This ABI looks like it will work fine for our use case. I don't think it
>>> has been mentioned yet, but we may still need multiple asm blocks
>>> for differing numbers of writes. For example, an array-based freelist push:
> 
>>> void push(void *obj) {
>>> if (index < maxlen) {
>>> freelist[index++] = obj;
>>> }
>>> }
> 
>>> would be more efficiently implemented with a two-write rseq_finish:
> 
>>> rseq_finish2(&freelist[index], obj, // first write
>>> &index, index + 1, // second write
>>> ...);
> 
>> Would pairing one rseq_start with two rseq_finish do the trick
>> there ?
> 
> Yes, two rseq_finish works, as long as the extra rseq management overhead
> is not substantial.

I've added a commit implementing rseq_finish2() in my rseq volatile
dev branch. You can fetch it at:

https://github.com/compudj/linux-percpu-dev/tree/rseq-fallback

I also have a separate test and benchmark tree in addition to the
kernel selftests here:

https://github.com/compudj/rseq-test

I named the first write a "speculative" write, and the second write
the "final" write.

Would you like to extend the test cases to cover your intended use-case ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com