linux-kernel - Re: [PATCH 2/3] ftrace: use struct pid

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Date:	Thu, 04 Dec 2008 05:40:00 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>,
	containers@...ts.osdl.org, Ingo Molnar <mingo@...e.hu>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] ftrace: use struct pid

Dave Hansen <dave@...ux.vnet.ibm.com> writes:

> On Thu, 2008-12-04 at 04:56 -0800, Dave Hansen wrote:
>> On Thu, 2008-12-04 at 04:42 -0800, Eric W. Biederman wrote:
>> > 
>> > > +static void clear_ftrace_pid_task(struct pid **pid)
>> > > +{
>> > > +     struct task_struct *p;
>> > > +
>> >         rcu_read_lock();
>> > 
>> > > +     do_each_pid_task(*pid, PIDTYPE_PID, p) {
>> > > +             clear_tsk_trace_trace(p);
>> > > +     } while_each_pid_task(*pid, PIDTYPE_PID, p);
>> >         rcu_read_unlock()
>> > 
>> > > +     put_pid(*pid);
>> > > +
>> > > +     *pid = NULL;
>> > > +}
>> 
>> Could we get away with sticking the rcu_read_{un}lock() inside those
>> macros?  Those are going to get used in pretty high level code and we're
>> allowed to nest rcu_read_lock().  No danger of deadlocks or lock
>> inversions.
>
> Why don't any of the other users of do_each_pid_task() use
> rcu_read_lock()?  They all seem to be under read_lock(&tasklist_lock)
> (except one is under a write lock of the same).

We probably should.  Historically read_lock(&tasklist_lock) implies
rcu_read_lock().  And the tasklist lock is what we hold when it is safe.

But if you look at find_vpid we should be holding just the rcu lock there.

Eric
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