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Message-ID: <20140814193700.GA9747@redhat.com> Date: Thu, 14 Aug 2014 21:37:00 +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 02:15 PM, Oleg Nesterov wrote: > > 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? > > You are right, the tree of processes only goes one way, > so there should be no deadlock in taking psig->stats_lock > and having thread_group_cputime_adjusted take sig->stats_lock > for read within that section. > > However, it might need some lockdep annotation to keep > lockdep from thinking we might the same lock recursively :) But wait_task_zombie() can (and should) call thread_group_cputime_adjusted(zombie_child) outside of parent's ->siglock or ->stats_lock so this this should be safe. 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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