linux-kernel - Re: Question about clearing of tsk->robust

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Date:	Tue, 15 Feb 2011 15:19:45 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Kenneth Albanowski (Palm GBU)" <Kenneth.Albanowski@...m.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>,
	Andreas Schwab <schwab@...ux-m68k.org>
Subject: Re: Question about clearing of tsk->robust_list in clone

On Tue, 15 Feb 2011, Peter Zijlstra wrote:

> On Tue, 2011-02-15 at 14:16 +0100, Thomas Gleixner wrote:
> > 
> > And I do not buy the argument about "complex glibc code" at all. glibc
> > already handles it for pthread_create() so why the hell can't it
> > handle it for fork() ? 
> 
> Going by comment #9 they think calling sys_set_robust_list() on every
> fork() is too expensive for them, but realistically we cannot do
> anything about it, ->robust_list is strictly task state.

Right. 

Vs. too expensive: We already call set_robust_list() on every
pthread_create and on every process start when libpthread is
linked. So what makes fork() so special? Aside of that
set_robust_list() is hardly an expensive syscall.

> I also think their suggestion in comment #11 (lazy state) is flawed,
> what if the parent never users robust futexes, in that case the state
> will indicate not to initialize the robust state for its children, again
> leading to the observed wreckage.

Yup.
 
> Realistically libpthread should register an on_fork() callback to ensure
> the state is properly propagated.

Makes sense.

Thanks,

	tglx
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