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Date: Wed, 4 Dec 2013 16:27:19 +0100 From: Oleg Nesterov <oleg@...hat.com> To: Frederic Weisbecker <fweisbec@...il.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, David Rientjes <rientjes@...gle.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Mandeep Singh Baines <msb@...omium.org>, "Ma, Xindong" <xindong.ma@...el.com>, Michal Hocko <mhocko@...e.cz>, Sameer Nanda <snanda@...omium.org>, Sergey Dyasly <dserrg@...il.com>, "Tu, Xiaobing" <xiaobing.tu@...el.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 1/4] introduce for_each_thread() to replace the buggy while_each_thread() On 12/04, Frederic Weisbecker wrote: > > On Wed, Dec 04, 2013 at 02:49:17PM +0100, Oleg Nesterov wrote: > > > > Yes, perhaps we will need for_each_thread_continue(). I am not sure > > yet. And note that, say, check_hung_uninterruptible_tasks() already > > does _continue if fact, although it is still not clear to me if we > > actually need this helper. > > So that's one of the possible users. _continue() can make sense if the > reader can easily cope with missing a few threads from time to time, which > is the case of the hung task detector. Yes, but again it is not clear if we need the new helper. For example. Note that you can simply do something like: // p can't go away rcu_read_lock(); for_each_thread(p, t) { do_something(t); if (need_to_sleep()) { get_task_struct(t); rcu_read_unlock(); schedule_timeout_interruptible(...); rcu_read_lock(); put_task_struct(); if (!pid_alive(t)) break; } } rcu_read_unlock(); If you rewrite this code using for_each_thread_continue (which is just list_for_each_entry_continue_rcu) the code will look more complex. > > Note also that _continue() can't be safely used lockless, unless > > you verify pid_alive() or something similar. > > Hmm, due to concurrent list_del()? > > Right, tsk->thread_list.next could point to junk after a list_del(), say if the next > entry has been freed. Yes. The same problem which while_each_thread() currently has (I mean, even ignoring the fact it is itself buggy). 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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