linux-kernel - Re: [PATCH V2] futex: set FLAGS_HAS_TIMEOUT during demux for FUTEX

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Date:	Thu, 14 Apr 2011 13:45:32 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH V2] futex: set FLAGS_HAS_TIMEOUT during demux for FUTEX_WAIT

On 04/14/2011 12:48 PM, Eric Dumazet wrote:
> Le jeudi 14 avril 2011 à 12:11 -0700, Darren Hart a écrit :
> 
>> I would say anything calling SYS_FUTEX is the futex slow path. The fast
>> path is cmpxchg in user space.
>>
> 
> Thats not a good reason to make it slower than necessary...
> 
>> It was. My thinking was that it was inconsistent to have the
>> FLAGS_HAS_TIMEOUT only available if a signal was received and a restart
>> was required. This is the only place it is currently needed, but the
>> inconsistency concerns me.
>>
> 
> I dont call this inconsistency, but right place for the code.
> 
>> How about the following, it reuses an existing if block and ensure the
>> FLAGS_HAS_TIMEOUT is always set if a timeout is used. It means the
>> FLAG_HAS_TIMEOUT is not available in the other futex_* routines with
>> timeouts (futex_lock_pi and futex_wait_requeue_pi), but they use absolute
>> timeouts and don't need it for restart - I can agree to that, although
>> I'm not keen on FLAG_HAS_TIMEOUT not being set whenever timeout is. That
>> could be added in the same way to the other functions if needed in the
>> future.
> 
> I dont understand why you insist setting in fast path a flag that is
> useless, unless we hit restart logic [ What I call the slow path in
> futex syscall ]

I'm not particularly attached to this approach, I just felt it made more
sense. Your initial objection was to the test in the do_futex(), so I
avoided the test by moving it into futex_wait().

The addition of an |= to an existing test block didn't seem significant
to me in this path. But, it isn't important enough to me to argue the point.

> 
> It seems more natural and efficient to me to go back to previous code.
> 
> Maybe rename FLAG_HAS_TIMEOUT to FLAG_HAS_TIMEOUT_ON_RESTART if you
> want, to make clear what is the meaning of this flag.
> 
> Now if you have plans to use this flag in futex code, outside of restart
> logic, please share them with us :)

Nope, no plans, and there is value in simply restoring the original
behavior, especially as this should go to stable as well. I've
resubmitted the patch with the "restart-block only approach" and
included "stable".

Thanks for the feedback,

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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