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Date:   Wed, 22 Aug 2018 09:03:17 +1000
From:   NeilBrown <neilb@...e.com>
To:     "Joel Fernandes \(Google\)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org
Cc:     kernel-team@...roid.com, Joel Fernandes <joel@...lfernandes.org>,
        willy@...radead.org, stable@...r.kernel.org, peterz@...radead.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: shmem: Correctly annotate new inodes

On Tue, Aug 14 2018, Joel Fernandes (Google) wrote:

> Directories and inodes don't necessarily need to be in the same
> lockdep class. For ex, hugetlbfs splits them out too to prevent
> false positives in lockdep. Annotate correctly after new inode
> creation. If its a directory inode, it will be put into a different
> class.
>
> This should fix a lockdep splat reported by syzbot:
>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 4.18.0-rc8-next-20180810+ #36 Not tainted
>> ------------------------------------------------------
>> syz-executor900/4483 is trying to acquire lock:
>> 00000000d2bfc8fe (&sb->s_type->i_mutex_key#9){++++}, at: inode_lock
>> include/linux/fs.h:765 [inline]
>> 00000000d2bfc8fe (&sb->s_type->i_mutex_key#9){++++}, at:
>> shmem_fallocate+0x18b/0x12e0 mm/shmem.c:2602
>>
>> but task is already holding lock:
>> 0000000025208078 (ashmem_mutex){+.+.}, at: ashmem_shrink_scan+0xb4/0x630
>> drivers/staging/android/ashmem.c:448
>>
>> which lock already depends on the new lock.
>>
>> -> #2 (ashmem_mutex){+.+.}:
>>        __mutex_lock_common kernel/locking/mutex.c:925 [inline]
>>        __mutex_lock+0x171/0x1700 kernel/locking/mutex.c:1073
>>        mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1088
>>        ashmem_mmap+0x55/0x520 drivers/staging/android/ashmem.c:361
>>        call_mmap include/linux/fs.h:1844 [inline]
>>        mmap_region+0xf27/0x1c50 mm/mmap.c:1762
>>        do_mmap+0xa10/0x1220 mm/mmap.c:1535
>>        do_mmap_pgoff include/linux/mm.h:2298 [inline]
>>        vm_mmap_pgoff+0x213/0x2c0 mm/util.c:357
>>        ksys_mmap_pgoff+0x4da/0x660 mm/mmap.c:1585
>>        __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
>>        __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
>>        __x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
>>        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #1 (&mm->mmap_sem){++++}:
>>        __might_fault+0x155/0x1e0 mm/memory.c:4568
>>        _copy_to_user+0x30/0x110 lib/usercopy.c:25
>>        copy_to_user include/linux/uaccess.h:155 [inline]
>>        filldir+0x1ea/0x3a0 fs/readdir.c:196
>>        dir_emit_dot include/linux/fs.h:3464 [inline]
>>        dir_emit_dots include/linux/fs.h:3475 [inline]
>>        dcache_readdir+0x13a/0x620 fs/libfs.c:193
>>        iterate_dir+0x48b/0x5d0 fs/readdir.c:51
>>        __do_sys_getdents fs/readdir.c:231 [inline]
>>        __se_sys_getdents fs/readdir.c:212 [inline]
>>        __x64_sys_getdents+0x29f/0x510 fs/readdir.c:212
>>        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #0 (&sb->s_type->i_mutex_key#9){++++}:
>>        lock_acquire+0x1e4/0x540 kernel/locking/lockdep.c:3924
>>        down_write+0x8f/0x130 kernel/locking/rwsem.c:70
>>        inode_lock include/linux/fs.h:765 [inline]
>>        shmem_fallocate+0x18b/0x12e0 mm/shmem.c:2602
>>        ashmem_shrink_scan+0x236/0x630 drivers/staging/android/ashmem.c:455
>>        ashmem_ioctl+0x3ae/0x13a0 drivers/staging/android/ashmem.c:797
>>        vfs_ioctl fs/ioctl.c:46 [inline]
>>        file_ioctl fs/ioctl.c:501 [inline]
>>        do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
>>        ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
>>        __do_sys_ioctl fs/ioctl.c:709 [inline]
>>        __se_sys_ioctl fs/ioctl.c:707 [inline]
>>        __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
>>        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> other info that might help us debug this:
>>
>> Chain exists of:
>>   &sb->s_type->i_mutex_key#9 --> &mm->mmap_sem --> ashmem_mutex
>>
>>  Possible unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(ashmem_mutex);
>>                                lock(&mm->mmap_sem);
>>                                lock(ashmem_mutex);
>>   lock(&sb->s_type->i_mutex_key#9);
>>
>>  *** DEADLOCK ***
>>
>> 1 lock held by syz-executor900/4483:
>>  #0: 0000000025208078 (ashmem_mutex){+.+.}, at:
>> ashmem_shrink_scan+0xb4/0x630 drivers/staging/android/ashmem.c:448
>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Cc: willy@...radead.org
> Cc: stable@...r.kernel.org
> Cc: peterz@...radead.org
> Suggested-by: Neil Brown <neilb@...e.com>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Reviewed-by: NeilBrown <neilb@...e.com>

This is necessary for any filesystem that doesn't use
unlock_new_inode().

(If/when you repost, I suggest including Andrew Morton).

Thanks,
NeilBrown


> ---
>  mm/shmem.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 2cab84403055..4429a8fd932d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2225,6 +2225,8 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
>  			mpol_shared_policy_init(&info->policy, NULL);
>  			break;
>  		}
> +
> +		lockdep_annotate_inode_mutex_key(inode);
>  	} else
>  		shmem_free_inode(sb);
>  	return inode;
> -- 
> 2.18.0.597.ga71716f1ad-goog

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