Re: [PATCH] perf: perf_event_exit_task_context:s/rcu_dereference/rcu_dereference_raw/
From: Oleg Nesterov
Date: Mon Jan 24 2011 - 07:00:04 EST
On 01/21, Paul E. McKenney wrote:
> On Fri, Jan 21, 2011 at 06:53:45PM +0100, Oleg Nesterov wrote:
> > But rcu_dereference_raw() looks a bit confusing, and it is not
> > very convenient to use read_barrier_depends() directly.
> > Paul, may be it makes sense to add the new trivial helper which
> > can be used instead?
> > Yes, this is only cosmetic issue, I know ;)
> Cosmetic issues can be pretty important to the poor guy trying to read
> the code. ;-)
> What keeps the structure that rcu_dereference_raw() returns a pointer
> to from going away?
It can't go away, current owns its ->perf_event_ctxp pointers. But
the pointer can be installed at any time by sys_perf_event_open().
Currently the code does
ctx = current->perf_event_ctxp[ctxn];
and in theory we need smp_read_barrier_depends() in between.
> Best would be if a lockdep condition could be
> constructed from the answer to this question and added to the appropriate
> rcu_dereference() primitive.
In this case the condition is "true", so we can use rcu_dereference_raw().
The only problem, it looks confusing. Especially because you actually
need rcu_read_lock() if you look at not_current_task->perf_event_ctxp.
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