linux-kernel - Re: [PATCH 4/4] exit: Lockless iteration over task list in mm_update_next

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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