Date:   Thu, 1 Sep 2016 20:15:17 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzkaller <syzkaller@...glegroups.com>
Cc:     eparis@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: fanotify: unkillable hanged processes

On Thu, Sep 1, 2016 at 8:04 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Sep 01, 2016 at 07:53:44PM +0200, Dmitry Vyukov wrote:
>> Hello,
>>
>> The following program:
>>
>> https://gist.githubusercontent.com/dvyukov/0952eeac71069b46b3fe0e28bd1a02bf/raw/396b9dcce2636cecab1a4161c15d3f066e6ef639/gistfile1.txt
>>
>> if run in a parallel loop creates unkillable hanged processes:
>>
>> -bash-4.3# ps afxu | grep a.out
>> root      4489  0.0  0.0   8868   340 ttyp1    S+   17:19   0:00
>>    \_ grep a.out
>> root      4423  0.0  0.0      0     0 ttyp0    D    17:16   0:00 [a.out]
>> root      4424  0.0  0.0      0     0 ttyp0    D    17:16   0:00 [a.out]
>> root      4425  0.0  0.0      0     0 ttyp0    D    17:16   0:00 [a.out]
>> root      4470  0.0  0.1   7016  2316 ttyp0    D    17:16   0:00
>> ./stress ./a.out
>>
>> This looks like a classical deadlock, but LOCKDEP is silent so
>> +LOCKDEP maintainers.
>
> fanotify_get_response() uses wait_event(), that's an asymmetric API and
> therefore one cannot include it in block-on chains. That is, while it
> blocks, it doesn't know who it blocks on etc..


Ah, I see. So that's deadlock on semaphores, that's not easily detectable.


>> Most of the processes are hanged at (from /proc/pid/stack):
>>
>> [<ffffffff814dd6c8>] __synchronize_srcu+0x248/0x380 kernel/rcu/srcu.c:448
>> [<ffffffff814dd81e>] synchronize_srcu+0x1e/0x40 kernel/rcu/srcu.c:492
>> [<ffffffff819448d7>] fsnotify_mark_destroy_list+0x107/0x370 fs/notify/mark.c:551
>> [<ffffffff8194163e>] fsnotify_destroy_group+0x1e/0xc0 fs/notify/group.c:57
>> [<ffffffff8194a84b>] fanotify_release+0x20b/0x2d0
>> fs/notify/fanotify/fanotify_user.c:392
>> [<ffffffff81868bbc>] __fput+0x28c/0x780 fs/file_table.c:208
>> [<ffffffff81869135>] ____fput+0x15/0x20 fs/file_table.c:244
>> [<ffffffff813ebf63>] task_work_run+0xf3/0x170 kernel/task_work.c:116
>> [<     inline     >] exit_task_work ./include/linux/task_work.h:21
>> [<ffffffff81394218>] do_exit+0x868/0x2e70 kernel/exit.c:828
>> [<ffffffff81396998>] do_group_exit+0x108/0x330 kernel/exit.c:958
>> [<     inline     >] SYSC_exit_group kernel/exit.c:969
>> [<ffffffff81396bdd>] SyS_exit_group+0x1d/0x20 kernel/exit.c:967
>> [<ffffffff86e10700>] entry_SYSCALL_64_fastpath+0x23/0xc1
>> arch/x86/entry/entry_64.S:208
>> [<ffffffffffffffff>] 0xffffffffffffffff
>>
>> One process holds the srcu lock:
>>
>> stress          D ffffffff86dfe09a 28024  4470      1 0x00000004
>>  ffff880064d9c440 0000000000000000 ffff8800686ab0c0 ffff88006cecc080
>>  ffff88006d322bd8 ffff8800653f7630 ffffffff86dfe09a ffff8800653f76e0
>>  0000000000000282 ffff88006d323568 ffff88006d323540 ffff880064d9c448
>> Call Trace:
>>  [<ffffffff86dff497>] schedule+0x97/0x1c0 kernel/sched/core.c:3414
>>  [<     inline     >] fanotify_get_response fs/notify/fanotify/fanotify.c:70
>>  [<ffffffff8194a347>] fanotify_handle_event+0x537/0x830
>> fs/notify/fanotify/fanotify.c:233
>>  [<     inline     >] send_to_group fs/notify/fsnotify.c:179
>>  [<ffffffff8193fc8d>] fsnotify+0x73d/0x1020 fs/notify/fsnotify.c:275
>>  [<     inline     >] fsnotify_perm ./include/linux/fsnotify.h:55
>>  [<ffffffff82acefb1>] security_file_open+0x151/0x190 security/security.c:887
>>  [<ffffffff8185a8db>] do_dentry_open+0x2ab/0xd30 fs/open.c:736
>>  [<ffffffff8185e445>] vfs_open+0x105/0x220 fs/open.c:860
>>  [<     inline     >] do_last fs/namei.c:3374
>>  [<ffffffff81895909>] path_openat+0x12f9/0x2ab0 fs/namei.c:3497
>>  [<ffffffff8189a7ac>] do_filp_open+0x18c/0x250 fs/namei.c:3532
>>  [<ffffffff81873528>] do_open_execat+0xe8/0x4d0 fs/exec.c:818
>>  [<ffffffff8187a18f>] do_execveat_common.isra.35+0x71f/0x1d80 fs/exec.c:1679
>>  [<     inline     >] do_execve fs/exec.c:1783
>>  [<     inline     >] SYSC_execve fs/exec.c:1864
>>  [<ffffffff8187c262>] SyS_execve+0x42/0x50 fs/exec.c:1859
>>  [<ffffffff810088ff>] do_syscall_64+0x1df/0x640 arch/x86/entry/common.c:288
>>  [<ffffffff86e107c3>] entry_SYSCALL64_slow_path+0x25/0x25
>> arch/x86/entry/entry_64.S:249
>>
>>
>> [  467.548485] Showing all locks held in the system:
>> [  467.548981] 2 locks held by bash/4044:
>> [  467.549313]  #0:  (&tty->ldisc_sem){.+.+.+}, at:
>> [<ffffffff86e0f7f7>] ldsem_down_read+0x37/0x40
>> [  467.550076]  #1:  (&ldata->atomic_read_lock){+.+...}, at:
>> [<ffffffff8322ec95>] n_tty_read+0x1e5/0x1860
>> [  467.550897] 3 locks held by bash/4062:
>> [  467.551210]  #0:  (sb_writers#5){.+.+.+}, at: [<ffffffff81863c04>]
>> vfs_write+0x3a4/0x4e0
>> [  467.551923]  #1:  (rcu_read_lock){......}, at: [<ffffffff83245c30>]
>> __handle_sysrq+0x0/0x4d0
>> [  467.552655]  #2:  (tasklist_lock){.+.+..}, at: [<ffffffff81496fc4>]
>> debug_show_all_locks+0x74/0x290
>> [  467.553454] 2 locks held by stress/4470:
>> [  467.553754]  #0:  (&sig->cred_guard_mutex){+.+.+.}, at:
>> [<ffffffff818799b3>] prepare_bprm_creds+0x53/0x110
>> [  467.554590]  #1:  (&fsnotify_mark_srcu){......}, at:
>> [<ffffffff8193f71d>] fsnotify+0x1cd/0x1020
>
> So not quite enough information, what is bash/4044's stack trace?
> Because that is holding the fanotify srcu reference.
>
> But it looks like a scenario where everyone is waiting for SRCU to
> complete, while the task holding up SRCU completion is waiting for
> something else.
>
> But I'm not at all familiar with fanotify.

4044 just holds a normal rcu_read_lock lock during sysrq processing.
I've sent the sysrq after the deadlock happened.
The bad guy is 4470, he holds fsnotify_mark_srcu.

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