linux-kernel - Re: [PATCH] ftrace: do_each_pid

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Message-ID: <m1fxivw673.fsf@fess.ebiederm.org>
Date:	Tue, 03 Feb 2009 13:39:44 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: do_each_pid_task() needs rcu lock

Oleg Nesterov <oleg@...hat.com> writes:

> On 02/03, Oleg Nesterov wrote:
>>
>> "ftrace: use struct pid" commit 978f3a45d9499c7a447ca7615455cefb63d44165
>> converted ftrace_pid_trace to "struct pid*". But we can't use
>> do_each_pid_task() without rcu_read_lock() even if we know the pid
>> itself can't go away (it was pinned in ftrace_pid_write). The exiting
>> task can detach itself from this pid at any moment.
>
> Q: why do we use do_each_pid_task(PIDTYPE_PID) ? We can never have more
> than 1 task in the loop. Perhaps,

That is a bug in do_each_pid_task(PIDTYPE_PID).
For ftrace we really want to grab all tasks with a given pid even
in the crazy exec case.

Eric
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