linux-kernel - Re: [RFC PATCH 00/11] sched: CFS low-latency features

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Message-ID: <alpine.LFD.2.00.1008270013390.2768@localhost.localdomain>
Date:	Fri, 27 Aug 2010 00:22:46 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC PATCH 00/11] sched: CFS low-latency features

On Thu, 26 Aug 2010, Thomas Gleixner wrote:
> On Thu, 26 Aug 2010, Peter Zijlstra wrote:
> > 
> > Fudging fork seems dubious at best, it seems generated by the use of
> > timer_create(.evp->sigev_notify = SIGEV_THREAD), which is a really
> > broken thing to do, it has very ill defined semantics and is utterly
> > unable to properly cope with error cases. Furthermore its trivial to
> > actually correctly implement the desired behaviour, so I'm really
> > skeptical on this front; friends don't let friends use SIGEV_THREAD.
> 
> SIGEV_THREAD is the best proof that the whole posix timer interface
> was comitte[e]d under the influence of not to be revealed
> mind-altering substances.
> 
> I completely object to add timer specific wakeup magic and support for
> braindead fork orgies to the kernel proper. All that mess can be fixed
> in user space by using sensible functionality.
> 
> Providing support for misdesigned crap just for POSIX compliance
> reasons and to make some of the blind abusers of that very same crap
> happy would be a completely stupid decision.
> 
> In fact that would make a brilliant precedence case for forcing the
> kernel to solve user space madness at the expense of kernel
> complexity. If we follow down that road we get requests for extra
> functionality for AIO, networking and whatever in a split second with
> no real good reason to reject them anymore.

I really risked eye cancer and digged into the glibc code.

	/* There is not much we can do if the allocation fails.  */
	(void) pthread_create (&th, &tk->attr, timer_sigev_thread, td);

So if the helper thread which gets the signal fails to create the
thread then everything is toast.

What about fixing the f*cked up glibc implementation in the first place
instead of fiddling in the kernel to support this utter madness? 

WTF can't the damned delivery thread not be created when timer_create
is called and the signal be delivered to that very thread directly via
SIGEV_THREAD_ID ?

Thanks,

	tglx
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