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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 24 Oct 2016 13:46:42 -0700
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Subject: Re: bio linked list corruption.
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Jones <davej@codemonkey.org.uk>, Chris Mason <clm@fb.com>,
        Andy Lutomirski <luto@kernel.org>, Jens Axboe <axboe@fb.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Josef Bacik <jbacik@fb.com>,
        David Sterba <dsterba@suse.com>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>
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On Mon, Oct 24, 2016 at 1:06 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> [69943.450108] Oops: 0003 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>
> This is an unhandled kernel page fault.  The string "Oops" is so helpful :-/

I think there was a line above it that DaveJ just didn't include.

>
>> [69943.454452] CPU: 1 PID: 21558 Comm: trinity-c60 Not tainted 4.9.0-rc1-think+ #11
>> [69943.463510] task: ffff8804f8dd3740 task.stack: ffffc9000b108000
>> [69943.468077] RIP: 0010:[<ffffffff810c3f6b>]
>> [69943.472704]  [<ffffffff810c3f6b>] __lock_acquire.isra.32+0x6b/0x8c0
>> [69943.477489] RSP: 0018:ffffc9000b10b9e8  EFLAGS: 00010086
>> [69943.482368] RAX: ffffffff81789b90 RBX: ffff8804f8dd3740 RCX: 0000000000000000
>> [69943.487410] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
>> [69943.492515] RBP: ffffc9000b10ba18 R08: 0000000000000001 R09: 0000000000000000
>> [69943.497666] R10: 0000000000000001 R11: 00003f9cfa7f4e73 R12: 0000000000000000
>> [69943.502880] R13: 0000000000000000 R14: ffffc9000af7bd48 R15: ffff8804f8dd3740
>> [69943.508163] FS:  00007f64904a2b40(0000) GS:ffff880507a00000(0000) knlGS:0000000000000000
>> [69943.513591] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [69943.518917] CR2: ffffffff81789d28 CR3: 00000004a8f16000 CR4: 00000000001406e0
>> [69943.524253] DR0: 00007f5b97fd4000 DR1: 0000000000000000 DR2: 0000000000000000
>> [69943.529488] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
>> [69943.534771] Stack:
>> [69943.540023]  ffff880507bd74c0
>> [69943.545317]  ffff8804f8dd3740 0000000000000046 0000000000000286[69943.545456]  ffffc9000af7bd08
>> [69943.550930]  0000000000000100 ffffc9000b10ba50 ffffffff810c4b68[69943.551069]  ffffffff810ba40c
>> [69943.556657]  ffff880400000000 0000000000000000 ffffc9000af7bd48[69943.556796] Call Trace:
>> [69943.562465]  [<ffffffff810c4b68>] lock_acquire+0x58/0x70
>> [69943.568354]  [<ffffffff810ba40c>] ? finish_wait+0x3c/0x70
>> [69943.574306]  [<ffffffff8178fef2>] _raw_spin_lock_irqsave+0x42/0x80
>> [69943.580335]  [<ffffffff810ba40c>] ? finish_wait+0x3c/0x70
>> [69943.586237]  [<ffffffff810ba40c>] finish_wait+0x3c/0x70
>> [69943.591992]  [<ffffffff81169727>] shmem_fault+0x167/0x1b0
>> [69943.597807]  [<ffffffff810ba6c0>] ? prepare_to_wait_event+0x100/0x100
>> [69943.603741]  [<ffffffff8117b46d>] __do_fault+0x6d/0x1b0
>> [69943.609743]  [<ffffffff8117f168>] handle_mm_fault+0xc58/0x1170
>> [69943.615822]  [<ffffffff8117e553>] ? handle_mm_fault+0x43/0x1170
>> [69943.621971]  [<ffffffff81044982>] __do_page_fault+0x172/0x4e0
>> [69943.628184]  [<ffffffff81044d10>] do_page_fault+0x20/0x70
>> [69943.634449]  [<ffffffff8132a897>] ? debug_smp_processor_id+0x17/0x20
>> [69943.640784]  [<ffffffff81791f3f>] page_fault+0x1f/0x30
>> [69943.647170]  [<ffffffff8133d69c>] ? strncpy_from_user+0x5c/0x170
>> [69943.653480]  [<ffffffff8133d686>] ? strncpy_from_user+0x46/0x170
>> [69943.659632]  [<ffffffff811f22a7>] setxattr+0x57/0x170
>> [69943.665846]  [<ffffffff8132a897>] ? debug_smp_processor_id+0x17/0x20
>> [69943.672172]  [<ffffffff810c1f09>] ? get_lock_stats+0x19/0x50
>> [69943.678558]  [<ffffffff810a58f6>] ? sched_clock_cpu+0xb6/0xd0
>> [69943.685007]  [<ffffffff810c40cf>] ? __lock_acquire.isra.32+0x1cf/0x8c0
>> [69943.691542]  [<ffffffff8132a8b3>] ? __this_cpu_preempt_check+0x13/0x20
>> [69943.698130]  [<ffffffff8109b9bc>] ? preempt_count_add+0x7c/0xc0
>> [69943.704791]  [<ffffffff811ecda1>] ? __mnt_want_write+0x61/0x90
>> [69943.711519]  [<ffffffff811f2638>] SyS_fsetxattr+0x78/0xa0
>> [69943.718300]  [<ffffffff8100255c>] do_syscall_64+0x5c/0x170
>> [69943.724949]  [<ffffffff81790a4b>] entry_SYSCALL64_slow_path+0x25/0x25
>> [69943.731521] Code:
>> [69943.738124] 00 83 fe 01 0f 86 0e 03 00 00 31 d2 4c 89 f7 44 89 45 d0 89 4d d4 e8 75 e7 ff ff 8b 4d d4 48 85 c0 44 8b 45 d0 0f 84 d8 02 00 00 <f0> ff 80 98 01 00 00 8b 15 e0 21 8f 01 45 8b 8f 50 08 00 00 85
>
> That's lock incl 0x198(%rax).  I think this is:
>
>     atomic_inc((atomic_t *)&class->ops);
>
> I suppose this could be stack corruption at work, but after a fair
> amount of staring, I still haven't found anything in the vmap_stack
> code that would cause stack corruption.

Well, it is intriguing that what faults is this:

                        finish_wait(shmem_falloc_waitq, &shmem_fault_wait);

where 'shmem_fault_wait' is a on-stack wait queue. So it really looks
very much like stack corruption.

What strikes me is that "finish_wait()" does this optimistic "has my
entry been removed" without holding the waitqueue lock (and uses
list_empty_careful() to make sure it does that "safely").

It has that big comment too:

                        /*
                         * shmem_falloc_waitq points into the shmem_fallocate()
                         * stack of the hole-punching task: shmem_falloc_waitq
                         * is usually invalid by the time we reach here, but
                         * finish_wait() does not dereference it in that case;
                         * though i_lock needed lest racing with wake_up_all().
                         */

the stack it comes from is the wait queue head from shmem_fallocate(),
which will do "wake_up_all()" under the inode lock.

On the face of it, the inode lock should make that safe and serialize
everything. And yes, finish_wait() does not touch the unsafe stuff if
the wait-queue (in the local stack) is empty, which wake_up_all()
*should* have guaranteed. It's just a regular wait-queue entry (that
DEFINE_WAIT() does that), so it uses the normal
autoremove_wake_function() that removes things on successful wakeup:

int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int
sync, void *key)
{
        int ret = default_wake_function(wait, mode, sync, key);

        if (ret)
                list_del_init(&wait->task_list);
        return ret;
}

So the only issue is "did default_wake_function() return true"? That's
try_to_wake_up(TASK_NORMAL, 0), and I note that it can return zero
(and thus *not* remove the entry - leavign the invalid entry tghere)
if

        if (!(p->state & state))
                goto out;

but "prepare_to_wait()" (which also ran with the inode->i_lock held,
and also takes the wait-queue lock) did set p->state to
TASK_UNINTERRUPTIBLE.

So this is all some really subtle code, but I'm not seeing that it
would be wrong.

            Linus