linux-kernel - Re: BUG: stuck on mmap

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Date:	Mon, 09 Feb 2015 11:34:32 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Konstantin Khlebnikov <koct9i@...il.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: BUG: stuck on mmap_sem in 3.18.6

On 02/09/2015 10:46 AM, Konstantin Khlebnikov wrote:
> On Mon, Feb 9, 2015 at 12:36 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> On 02/09/2015 08:14 AM, Konstantin Khlebnikov wrote:
>>> Python was running under ptrace-based sandbox "sydbox" used exherbo
>>> chroot. Kernel: 3.18.6 + my patch "mm: prevent endless growth of
>>> anon_vma hierarchy" (patch seems stable).
>>>
>>> [ 4674.087780] INFO: task python:25873 blocked for more than 120 seconds.
>>> [ 4674.087793]       Tainted: G     U         3.18.6-zurg+ #158
>>> [ 4674.087797] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
>>> disables this message.
>>> [ 4674.087801] python          D ffff88041e2d2000 14176 25873  25630 0x00000102
>>> [ 4674.087817]  ffff880286247b68 0000000000000086 ffff8803d5fe6b40
>>> 0000000000012000
>>> [ 4674.087824]  ffff880286247fd8 0000000000012000 ffff88040c16eb40
>>> ffff8803d5fe6b40
>>> [ 4674.087830]  0000000300000003 ffff8803d5fe6b40 ffff880362888e78
>>> ffff880362888e60
>>> [ 4674.087836] Call Trace:
>>> [ 4674.087854]  [<ffffffff81696be9>] schedule+0x29/0x70
>>> [ 4674.087865]  [<ffffffff81699815>] rwsem_down_write_failed+0x1d5/0x2f0
>>> [ 4674.087873]  [<ffffffff812d4c73>] call_rwsem_down_write_failed+0x13/0x20
>>> [ 4674.087881]  [<ffffffff816990c1>] ? down_write+0x31/0x50
>>> [ 4674.087891]  [<ffffffff811f3b44>] do_coredump+0x144/0xee0
>>> [ 4674.087900]  [<ffffffff810b66f7>] ? pick_next_task_fair+0x397/0x450
>>> [ 4674.087909]  [<ffffffff810026a6>] ? __switch_to+0x1d6/0x5f0
>>> [ 4674.087915]  [<ffffffff816966e6>] ? __schedule+0x3a6/0x880
>>> [ 4674.087924]  [<ffffffff81690000>] ? klist_remove+0x40/0xd0
>>> [ 4674.087932]  [<ffffffff81093988>] get_signal+0x298/0x6b0
>>> [ 4674.087940]  [<ffffffff81003588>] do_signal+0x28/0xbb0
>>> [ 4674.087946]  [<ffffffff8109276d>] ? do_send_sig_info+0x5d/0x80
>>> [ 4674.087955]  [<ffffffff81004179>] do_notify_resume+0x69/0xb0
>>> [ 4674.087963]  [<ffffffff8169b028>] int_signal+0x12/0x17
>>>
>>> Maybe this guy did something wrong?
>>
>> Well he has do_coredump on stack, so he did something wrong in userspace? But
>> here he's just waiting on down_write. Unless there's some bug in do_coredump
>> that would lock for read and then for write, without an unlock in between?
> 
> I mean khugepaged. This code looks really messy. Maybe it already has
> mmap_sem locked for read and tries to lock it again:

Yeah it is messy, and incidentally I'm trying to change how it works. But I
didn't find such double lock bug there.

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