Re: possible deadlock in start_this_handle (2)

[Jan Kara]

Hello,

added mm guys to CC.

On Wed 10-02-21 05:35:18, syzbot wrote:
> HEAD commit:    1e0d27fc Merge branch 'akpm' (patches from Andrew)
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15cbce90d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=bd1f72220b2e57eb
> dashboard link: https://syzkaller.appspot.com/bug?extid=bfdded10ab7dcd7507ae
> userspace arch: i386
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+bfdded10ab7dcd7507ae@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.11.0-rc6-syzkaller #0 Not tainted
> ------------------------------------------------------
> kswapd0/2246 is trying to acquire lock:
> ffff888041a988e0 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0xf81/0x1380 fs/jbd2/transaction.c:444
> 
> but task is already holding lock:
> ffffffff8be892c0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x0/0x30 mm/page_alloc.c:5195
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #2 (fs_reclaim){+.+.}-{0:0}:
>        __fs_reclaim_acquire mm/page_alloc.c:4326 [inline]
>        fs_reclaim_acquire+0x117/0x150 mm/page_alloc.c:4340
>        might_alloc include/linux/sched/mm.h:193 [inline]
>        slab_pre_alloc_hook mm/slab.h:493 [inline]
>        slab_alloc_node mm/slub.c:2817 [inline]
>        __kmalloc_node+0x5f/0x430 mm/slub.c:4015
>        kmalloc_node include/linux/slab.h:575 [inline]
>        kvmalloc_node+0x61/0xf0 mm/util.c:587
>        kvmalloc include/linux/mm.h:781 [inline]
>        ext4_xattr_inode_cache_find fs/ext4/xattr.c:1465 [inline]
>        ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1508 [inline]
>        ext4_xattr_set_entry+0x1ce6/0x3780 fs/ext4/xattr.c:1649
>        ext4_xattr_ibody_set+0x78/0x2b0 fs/ext4/xattr.c:2224
>        ext4_xattr_set_handle+0x8f4/0x13e0 fs/ext4/xattr.c:2380
>        ext4_xattr_set+0x13a/0x340 fs/ext4/xattr.c:2493
>        ext4_xattr_user_set+0xbc/0x100 fs/ext4/xattr_user.c:40
>        __vfs_setxattr+0x10e/0x170 fs/xattr.c:177
>        __vfs_setxattr_noperm+0x11a/0x4c0 fs/xattr.c:208
>        __vfs_setxattr_locked+0x1bf/0x250 fs/xattr.c:266
>        vfs_setxattr+0x135/0x320 fs/xattr.c:291
>        setxattr+0x1ff/0x290 fs/xattr.c:553
>        path_setxattr+0x170/0x190 fs/xattr.c:572
>        __do_sys_setxattr fs/xattr.c:587 [inline]
>        __se_sys_setxattr fs/xattr.c:583 [inline]
>        __ia32_sys_setxattr+0xbc/0x150 fs/xattr.c:583
>        do_syscall_32_irqs_on arch/x86/entry/common.c:77 [inline]
>        __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:139
>        do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:164
>        entry_SYSENTER_compat_after_hwframe+0x4d/0x5c

This stacktrace should never happen. ext4_xattr_set() starts a transaction.
That internally goes through start_this_handle() which calls:

	handle->saved_alloc_context = memalloc_nofs_save();

and we restore the allocation context only in stop_this_handle() when
stopping the handle. And with this fs_reclaim_acquire() should remove
__GFP_FS from the mask and not call __fs_reclaim_acquire().

Now I have no idea why something here didn't work out. Given we don't have
a reproducer it will be probably difficult to debug this. I'd note that
about year and half ago similar report happened (got autoclosed) so it may
be something real somewhere but it may also be just some HW glitch or
something like that.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR