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Date: Mon, 6 Feb 2017 11:39:19 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [RFC PATCH 1/2] mm, vmscan: account the number of isolated pages
 per zone
Message-ID: <20170206103918.GD3097@dhcp22.suse.cz>
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, peterz@infradead.org
Cc: hch@lst.de, mgorman@suse.de, viro@ZenIV.linux.org.uk, linux-mm@kvack.org, hannes@cmpxchg.org, linux-kernel@vger.kernel.org

On Sun 05-02-17 19:43:07, Tetsuo Handa wrote:
> Michal Hocko wrote:
> I got same warning with ext4. Maybe we need to check carefully.
> 
> [  511.215743] =====================================================
> [  511.218003] WARNING: RECLAIM_FS-safe -> RECLAIM_FS-unsafe lock order detected
> [  511.220031] 4.10.0-rc6-next-20170202+ #500 Not tainted
> [  511.221689] -----------------------------------------------------
> [  511.223579] a.out/49302 [HC0[0]:SC0[0]:HE1:SE1] is trying to acquire:
> [  511.225533]  (cpu_hotplug.dep_map){++++++}, at: [<ffffffff810a1477>] get_online_cpus+0x37/0x80
> [  511.227795] 
> [  511.227795] and this task is already holding:
> [  511.230082]  (jbd2_handle){++++-.}, at: [<ffffffff813a8be7>] start_this_handle+0x1a7/0x590
> [  511.232592] which would create a new lock dependency:
> [  511.234192]  (jbd2_handle){++++-.} -> (cpu_hotplug.dep_map){++++++}
> [  511.235966] 
> [  511.235966] but this new dependency connects a RECLAIM_FS-irq-safe lock:
> [  511.238563]  (jbd2_handle){++++-.}
> [  511.238564] 
> [  511.238564] ... which became RECLAIM_FS-irq-safe at:
> [  511.242078]   
> [  511.242084] [<ffffffff811089db>] __lock_acquire+0x34b/0x1640
> [  511.244495] [<ffffffff8110a119>] lock_acquire+0xc9/0x250
> [  511.246697] [<ffffffff813b3525>] jbd2_log_wait_commit+0x55/0x1d0
[...]
> [  511.276216] to a RECLAIM_FS-irq-unsafe lock:
> [  511.278128]  (cpu_hotplug.dep_map){++++++}
> [  511.278130] 
> [  511.278130] ... which became RECLAIM_FS-irq-unsafe at:
> [  511.281809] ...
> [  511.281811]   
> [  511.282598] [<ffffffff81108141>] mark_held_locks+0x71/0x90
> [  511.284854] [<ffffffff8110ab6f>] lockdep_trace_alloc+0x6f/0xd0
> [  511.287218] [<ffffffff812744c8>] kmem_cache_alloc_node_trace+0x48/0x3b0
> [  511.289755] [<ffffffff810cfa65>] __smpboot_create_thread.part.2+0x35/0xf0
> [  511.292329] [<ffffffff810d0026>] smpboot_create_threads+0x66/0x90
[...]
> [  511.317867] other info that might help us debug this:
> [  511.317867] 
> [  511.320920]  Possible interrupt unsafe locking scenario:
> [  511.320920] 
> [  511.323218]        CPU0                    CPU1
> [  511.324622]        ----                    ----
> [  511.325973]   lock(cpu_hotplug.dep_map);
> [  511.327246]                                local_irq_disable();
> [  511.328870]                                lock(jbd2_handle);
> [  511.330483]                                lock(cpu_hotplug.dep_map);
> [  511.332259]   <Interrupt>
> [  511.333187]     lock(jbd2_handle);

Peter, is there any way how to tell the lockdep that this is in fact
reclaim safe? The direct reclaim only does the trylock and backs off so
we cannot deadlock here.

Or am I misinterpreting the trace?
-- 
Michal Hocko
SUSE Labs

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