Re: [RFC,PATCH 2/2] cputimers/proc:do_task_stat()->thread_group_times() is racy and O(n) under ->siglock

[Peter Zijlstra]

On Wed, 2010-03-24 at 21:45 +0100, Oleg Nesterov wrote:
> Nowadays ->siglock is overloaded, it would be really nice to change
> do_task_stat() to walk through the list of threads lockless. And note
> that we are doing while_each_thread() twice!
> 
> while_each_thread() is rcu-safe, but thread_group_times() also needs
> ->siglock to serialize the modifications of signal_struct->prev_Xtime
> members.
> 
> 

Right, so from what I remember the issue is that, yes top et al rely on
that monotonicity, but more importantly I think
clock_gettime(CLOCK_PROCESS_CPUTIME_ID) should indeed use ->siglock to
ensure it serializes against do_exit() so that either we iterate the
thread or get the accumulated runtime from signal_struct but not both
(or neither).



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