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Date: Thu, 26 Apr 2018 19:04:14 +0300 From: Kirill Tkhai <ktkhai@...tuozzo.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Andrea Parri <andrea.parri@...rulasolutions.com>, akpm@...ux-foundation.org, oleg@...hat.com, viro@...iv.linux.org.uk, mingo@...nel.org, paulmck@...ux.vnet.ibm.com, keescook@...omium.org, riel@...hat.com, mhocko@...e.com, tglx@...utronix.de, kirill.shutemov@...ux.intel.com, marcos.souza.org@...il.com, hoeun.ryu@...il.com, pasha.tatashin@...cle.com, gs051095@...il.com, ebiederm@...ssion.com, dhowells@...hat.com, rppt@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org, Alan Stern <stern@...land.harvard.edu>, Will Deacon <will.deacon@....com>, Boqun Feng <boqun.feng@...il.com> Subject: Re: [PATCH 4/4] exit: Lockless iteration over task list in mm_update_next_owner() On 26.04.2018 18:20, Peter Zijlstra wrote: > On Thu, Apr 26, 2018 at 04:52:39PM +0300, Kirill Tkhai wrote: >>>> >>>> 1)for_each_process(g) copy_process() >>>> p->mm = mm >>>> smp_rmb(); smp_wmb() implied by alloc_pid() >>>> if (g->flags & PF_KTHREAD) list_add_tail_rcu(&p->tasks, &init_task.tasks) >>>> >>>> 2)for_each_thread(g, c) copy_process() >>>> p->mm = mm >>>> smp_rmb(); smp_wmb() implied by alloc_pid() >>>> tmp = READ_ONCE(c->mm) list_add_tail_rcu(&p->thread_node, ...) > > For these two; what's the purpose of the smp_rmb()? which loads are > ordered? (1) and (2) make visible g->flags and c->mm in case we see process is linked to the task list. It seems in mm_update_next_owner() we may see result of list_add_tail_rcu(&p->thread_node, ...), but p->mm = mm will not be visible. Hm, also I've dived into list_add_tail_rcu() and it contains smp_wmb(). So, it seems we do not need the barrier implied by alloc_pid(). Kirill
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