linux-kernel - Re: linux missing support for _POSIX_THREAD

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Date:	Wed, 27 May 2009 14:00:16 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: linux missing support for _POSIX_THREAD_CPUTIME?

Thomas Gleixner wrote:
> On Wed, 27 May 2009, Chris Friesen wrote:
> 
> 
>>Thomas Gleixner wrote:
>>
>>>On Wed, 27 May 2009, Chris Friesen wrote:
>>>
>>>That's more a question for the glibc folks. CC'ed.
>>
>>Actually, I'm looking at the kernel code here.  It looks like
>>posix_cpu_clock_get() should be able to handle a per-task cpu clock for
>>another thread within the same process.  However, it's only ever called
>>by thread_cpu_clock_get(), which hardcodes a clock_id of THREAD_CLOCK,
>>which corresponds to the current thread.
> 
> 
> No, it's also called via the CLOCK_DISPATCH magic in posix_timers.c

>>Similarly, invalid_clockid() will say that the clock_id is invalid if
>>it's positive but greater than 15.  If my math is right, this means that
>>any pid > 2 will result in invalid_clockid() failing.

Both invalid_clockid() and CLOCK_DISPATCH assume that the clock_id will
be negative in order to call posix_cpu_clock_get().

In pthread_getcpuclockid() in glibc, I couldn't see how we were ending
up with a negative clock_id.  However, I finally clued in to the fact
that there are multiple implementations of this function, and in one
code path in one of the implementations we call MAKE_THREAD_CPUCLOCK()
which can result in a negative clock_id.

Thanks for the help.

Chris
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