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Message-ID: <20140814181542.GB5091@redhat.com> Date: Thu, 14 Aug 2014 20:15:42 +0200 From: Oleg Nesterov <oleg@...hat.com> To: Rik van Riel <riel@...hat.com> Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>, Frank Mayhar <fmayhar@...gle.com>, Frederic Weisbecker <fweisbec@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Sanjay Rao <srao@...hat.com>, Larry Woodman <lwoodman@...hat.com> Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock On 08/14, Rik van Riel wrote: > > On 08/14/2014 12:12 PM, Oleg Nesterov wrote: > > > > Or you can expand the scope of write_seqlock/write_sequnlock, so that > > __unhash_process in called from inside the critical section. This looks > > simpler at first glance. > > The problem with that is that wait_task_zombie() calls > thread_group_cputime_adjusted() in that if() branch, and > that code ends up taking the seqlock for read... Not sure I understand... This modifies parent->signal->c* counters, and obviously the exiting thread is not the member of parent's thread group, so thread_group_cputime_adjusted(parent) can never account the exiting child twice simply because it won't see it? > However, in __exit_signal that approach should work. Yes, > > Hmm, wait, it seems there is yet another problem ;) Afaics, you also > > need to modify __exit_signal() so that ->sum_sched_runtime/etc are > > accounted unconditionally, even if the group leader exits. > > > > Probably this is not a big problem, and sys_times() or clock_gettime() > > do not care at all because they use current. > > > > But without this change thread_group_cputime(reaped_zombie) won't look > > at this task_struct at all, this can lead to non-monotonic result if > > it was previously called when this task was alive (non-reaped). > > You mean this whole block needs to run regardless of whether > the group is dead? > > task_cputime(tsk, &utime, &stime); > write_seqlock(&sig->stats_lock); > sig->utime += utime; > sig->stime += stime; > sig->gtime += task_gtime(tsk); > sig->min_flt += tsk->min_flt; > sig->maj_flt += tsk->maj_flt; > sig->nvcsw += tsk->nvcsw; > sig->nivcsw += tsk->nivcsw; > sig->inblock += task_io_get_inblock(tsk); > sig->oublock += task_io_get_oublock(tsk); > task_io_accounting_add(&sig->ioac, &tsk->ioac); > sig->sum_sched_runtime += tsk->se.sum_exec_runtime; Yes. > How does that square with wait_task_zombie reaping the > statistics of the whole group with thread_group_cputime_adjusted() > when the group leader is exiting? Again, not sure I understand... thread_group_cputime_adjusted() in wait_task_zombie() is fine in any case. Nobody but us can reap this zombie. It seems that we misunderstood each other, let me try again. Just to simplify, suppose we have, say, sys_times_by_pid(pid, ...) { rcu_read_lock(); task = find_task_by_vpid(pid); if (task) get_task_struct(task); rcu_read_unlock(); if (!task) return -ESRCH; thread_group_cputime(task, ...); copy_to_user(); return 0; } Note that this task can exit right after rcu_read_unlock(), and it can be also reaped (by its parent or by itself) and removed from the thread list. In this case for_each_thread() will see no threads, and thus it will only read task->signal->*time. This means that sys_times_by_pid() can simply return the wrong result instead of failure. Say, It can even return "all zeros" if this task was single-threaded. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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