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

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Message-ID: <CO1PR15MB0982484D57E73C83E6E6DB75DD1F0@CO1PR15MB0982.namprd15.prod.outlook.com>
Date:	Fri, 12 Aug 2016 20:05:17 +0000
From:	Dave Watson <davejwatson@...com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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


>>>> 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 ?
>>
>
>Hi Dave!
>
>I just pushed a rseq_finish2() test in my rseq-fallback branch. It implements
>a per-cpu buffer holding pointers, and pushes/pops items to/from it.
>
>To use it:
>
>cd tools/testing/selftests/rseq
>./param_test -T b
>
>(see -h for advanced usage)
>
>Let me know if I got it right!

Hi Mathieu,

Thanks, you beat me to it.    I commented on the github, that's pretty much it.  

> In the kernel, if rather than testing for:
> 
> if ((void __user *)instruction_pointer(regs) < post_commit_ip) {
> 
> we could test for both start_ip and post_commit_ip:
> 
> if ((void __user *)instruction_pointer(regs) < post_commit_ip
>     && (void __user *)instruction_pointer(regs) >= start_ip) {
> 
> We could perform the failure path (storing NULL into the rseq_cs
> field of struct rseq) in C rather than being required to do it in
> assembly at addresses >= to post_commit_ip, all because the kernel
> would test whether we are within the assembly block address range
> using both the lower and upper bounds (start_ip and post_commit_ip).

Sounds reasonable to me.  I agree it would be best to move the failure path 
out of the asm if possible.