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Date:   Wed, 16 Dec 2020 15:33:56 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     syzbot <syzbot+c9937dfb2303a5f18640@...kaller.appspotmail.com>,
        io-uring@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...nel.org, mingo@...hat.com,
        peterz@...radead.org, rostedt@...dmis.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        will@...nel.org, Tejun Heo <tj@...nel.org>
Subject: Re: WARNING in percpu_ref_kill_and_confirm (2)

On 12/16/20 2:46 PM, Jens Axboe wrote:
> On 12/16/20 2:14 PM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    7b1b868e Merge tag 'for-linus' of git://git.kernel.org/pub..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1156046b500000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=3416bb960d5c705d
>> dashboard link: https://syzkaller.appspot.com/bug?extid=c9937dfb2303a5f18640
>> compiler:       gcc (GCC) 10.1.0-syz 20200507
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1407c287500000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10ed5f07500000
>>
>> The issue was bisected to:
>>
>> commit 4d004099a668c41522242aa146a38cc4eb59cb1e
>> Author: Peter Zijlstra <peterz@...radead.org>
>> Date:   Fri Oct 2 09:04:21 2020 +0000
>>
>>     lockdep: Fix lockdep recursion
> 
> Ehhh no... This is timing dependent, so probably why it ends up pinpointing
> something totally unrelated.
> 
>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14e9d433500000
>> final oops:     https://syzkaller.appspot.com/x/report.txt?x=16e9d433500000
>> console output: https://syzkaller.appspot.com/x/log.txt?x=12e9d433500000
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+c9937dfb2303a5f18640@...kaller.appspotmail.com
>> Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion")
>>
>> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000441309
>> RDX: 0000000000000002 RSI: 00000000200000c0 RDI: 0000000000003ad1
>> RBP: 000000000000f2ae R08: 0000000000000002 R09: 00000000004002c8
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004021d0
>> R13: 0000000000402260 R14: 0000000000000000 R15: 0000000000000000
>> ------------[ cut here ]------------
>> percpu_ref_kill_and_confirm called more than once on io_ring_ctx_ref_free!
>> WARNING: CPU: 0 PID: 8476 at lib/percpu-refcount.c:382 percpu_ref_kill_and_confirm+0x126/0x180 lib/percpu-refcount.c:382
>> Modules linked in:
>> CPU: 0 PID: 8476 Comm: syz-executor389 Not tainted 5.10.0-rc7-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> RIP: 0010:percpu_ref_kill_and_confirm+0x126/0x180 lib/percpu-refcount.c:382
>> Code: 5d 08 48 8d 7b 08 48 89 fa 48 c1 ea 03 80 3c 02 00 75 5d 48 8b 53 08 48 c7 c6 00 4b 9d 89 48 c7 c7 60 4a 9d 89 e8 c6 97 f6 04 <0f> 0b 48 b8 00 00 00 00 00 fc ff df 48 89 ea 48 c1 ea 03 80 3c 02
>> RSP: 0018:ffffc9000b94fe10 EFLAGS: 00010086
>> RAX: 0000000000000000 RBX: ffff888011da4580 RCX: 0000000000000000
>> RDX: ffff88801fe84ec0 RSI: ffffffff8158c835 RDI: fffff52001729fb4
>> RBP: ffff88801539f000 R08: 0000000000000001 R09: ffff8880b9e2011b
>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000293
>> R13: 0000000000000000 R14: 0000000000000000 R15: ffff88802de28758
>> FS:  00000000014ab880(0000) GS:ffff8880b9e00000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007f2a7046b000 CR3: 0000000023368000 CR4: 0000000000350ef0
>> Call Trace:
>>  percpu_ref_kill include/linux/percpu-refcount.h:149 [inline]
>>  io_ring_ctx_wait_and_kill+0x2b/0x450 fs/io_uring.c:8382
>>  io_uring_release+0x3e/0x50 fs/io_uring.c:8420
>>  __fput+0x285/0x920 fs/file_table.c:281
>>  task_work_run+0xdd/0x190 kernel/task_work.c:151
>>  tracehook_notify_resume include/linux/tracehook.h:188 [inline]
>>  exit_to_user_mode_loop kernel/entry/common.c:164 [inline]
>>  exit_to_user_mode_prepare+0x17e/0x1a0 kernel/entry/common.c:191
>>  syscall_exit_to_user_mode+0x38/0x260 kernel/entry/common.c:266
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> RIP: 0033:0x441309
>> Code: e8 5c ae 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 3b 0a fc ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007ffed6545d38 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
>> RAX: fffffffffffffff4 RBX: 0000000000000000 RCX: 0000000000441309
>> RDX: 0000000000000002 RSI: 00000000200000c0 RDI: 0000000000003ad1
>> RBP: 000000000000f2ae R08: 0000000000000002 R09: 00000000004002c8
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004021d0
>> R13: 0000000000402260 R14: 0000000000000000 R15: 0000000000000000
> 
> What I think happens here is that we switch mode, but exit before it's
> done. Wonder if there's a wait to wait for that on exit. Tejun?

Sorry, that was another report... Looks like this one is exercising
memory failure, maybe there's an issue with the teardown path for
certain failures. I'll take a look.

-- 
Jens Axboe

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