Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Ben Maurer <bmaurer@fb.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-api <linux-api@vger.kernel.org>
Subject: Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call
Date: Mon, 23 Oct 2017 20:44:01 +0000 (UTC)	[thread overview]
Message-ID: <439398759.47028.1508791441765.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CY4PR15MB16888F91F41A4A1D322C102CCF460@CY4PR15MB1688.namprd15.prod.outlook.com>

----- On Oct 23, 2017, at 7:30 PM, Ben Maurer bmaurer@fb.com wrote:

>> if (!((long)ip - (long)start_ip <= (long)post_commit_offset))
>>   return 1;
> 
>> This introduces an issue here: if "ip" is lower than "start_ip", we
>> can incorrectly think we are in a critical section, when we are in
>> fact not.
> 
> This shouldn't be an issue if we used unsigned numbers. Eg if start_ip is X and
> post_commit_offset is L, then (ip - X <= L) means that if ip is less than X ip
> - X will be signed, which will become a large unsigned value.
> 
>> or to the kernel to set it back to NULL if it finds out that it is
>> preempting/delivering a signal over an instruction pointer outside
>> of the current rseq_cs start_ip/post_commit_ip range (lazy clear).
> 
> I see, lazy clear makes sense. Still, if during most execution periods the user
> code enters some rseq section (likely if rseq is used for something like
> malloc) on every context switch this code will have to be run.
> 
>> Moreover, this modification would add a subtraction on the common case
>> (ip - start_ip), and makes the ABI slightly uglier.
> 
> We could benchmark it but the subtraction should be similar in cost to the extra
> comparison but reducing the number of branches seems like it will help as well.
> FWIW GCC attempts to translate this kind of sequence to a subtract and compare:
> https://godbolt.org/g/5DGLvo.
> 
> I agree the ABI is uglier, but since we're mucking with every context switch I
> thought I'd point it out.

Thanks for following up on this. I did not initially realize the importance
of doing the unsigned comparison. I've pushed a commit in my private dev branch
implementing your suggestion.

https://github.com/compudj/linux-percpu-dev/commit/4cf8e9104636b51741c0118f2c88519e3acab7aa

Thanks!

Mathieu

> 
>> If I understand well, you are proposing to speed up .so load time by
>> means of removing relocations of pointers within rseq_cs, done by
>> making those relative to the rseq_cs address.
> 
> Yeah, I think this may be overkill as optimization.

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