linux-kernel - Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of ->signal to check the task is alive

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Date:	Wed,  4 Feb 2009 19:31:20 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Lin Ming <ming.m.lin@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of ->signal to
	check the task is alive

> It doesn't matter which pointer to check under tasklist to ensure the task
> was not released, ->signal or ->sighand. But we are going to make ->signal
> refcountable, change the code to use ->sighand.

I haven't been following what that's about (signal_struct already has two
atomic counts!).  Uses here protecting cpu_clock_sample_group() e.g., are
around looking at ->signal->foobar, so if ->signal is still there, why not
look at it and be able to get the sample in whatever small window this is?

I don't really understand what this new case might mean though.  Most
things that look at ->signal need to lock it, so access doesn't make any
sense if there is no siglock because ->sighand is clear while ->signal is not.

Thanks,
Roland
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