Re: [PATCH] futex: bugfix for robust futex deadlock when waking only one thread in handle_futex_death

From: Darren Hart <dvhart@linux.intel.com>
To: zhang.yi20@zte.com.cn
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] futex: bugfix for robust futex deadlock when waking only one thread in handle_futex_death
Date: Thu, 18 Apr 2013 07:54:30 -0700	[thread overview]
Message-ID: <51700926.8020309@linux.intel.com> (raw)
In-Reply-To: <OF1AAB4598.9A9CCAE8-ON48257B51.000416F3-48257B51.0009DE5A@zte.com.cn>

On 04/17/2013 06:47 PM, zhang.yi20@zte.com.cn wrote:
> 
> 
> Darren Hart <dvhart@linux.intel.com> wrote on 2013/04/18 03:42:07:
> 
>> 
>> 
>> On 04/17/2013 03:40 AM, zhang.yi20@zte.com.cn wrote:
>> > Darren Hart <dvhart@linux.intel.com> wrote on 2013/04/17 01:05:28:
>> > 
>> > 
>> >>
>> >> Performance isn't an issue here as this is an error path. The question
>> >> is if the
>> >> changed behavior will constitute a problem for existing applications.
>> > Rather
>> >> than a serialized cascading wake, we have them all wake at once. If an
>> >> application depended on the first waker after owner death to do some
>> > cleanup
>> >> before the rest came along, I could imagine some potential for failure
>> >> there.
>> >>
>> > 
>> > I don't find out there are any APIs can wake all waiters at once, so still
>> > use futex_wake.
>> > When waiter return form futex_wait syscall, glibc check the futex's value
>> > and try to modify it by using atomic instructions, and let the waiter
>> > return only if successed.
>> > The applications which not use the glibc's library should follow this.
>> 
>> Indeed they *should*. :-)
>> 
>> > 
>> >> One possible alternative would be to wake waiters for a different
>> >> process group
>> >> when OWNER_DEAD is set, and leave it as a single wake.
>> >>
>> > 
>> > To wake one waiter of other process cannot slove this problem , because it
>> > can be exiting too.
>> 
>> If I understood the point of your change, it was to ensure all tasks
>> would be woken because tasks that were exiting wouldn't propogate the
>> wake. If there are nothing but exiting tasks available.... does it even
>> matter?
>> 
> 
> I mean that there may be some processes (more than 2) waitting for the
> lock, we can't choose the one which is exiting or it will exit later.
> It's difficult to accomplish this.
> 

"or it will exit later" .... I don't follow you there, it sounds like
you are saying if we try to wake the exiting process, that process will
be delayed and take longer to exit.... I don't think that is what you
meant. Can you elaborate please?

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