9p2000.L stat/unlink race (WARNING: fs/inode.c:280 drop_nlink)

[Vegard Nossum]

Hi,

I've been seeing this:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 2377 at fs/inode.c:280 drop_nlink+0xc2/0x120
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 2377 Comm: syz-executor Not tainted 4.8.0-rc2+ #145
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
 0000000000000000 ffff880116437b50 ffffffff81f9f201 ffffffff83e72d60
 ffff880116437c28 ffffffff83ef6100 ffffffff847070c0 ffff880116437c18
 ffffffff8150538a 0000000041b58ab3 ffffffff844dc5dd ffffffff815051a7
Call Trace:
 [<ffffffff81f9f201>] dump_stack+0x83/0xb2
 [<ffffffff8150538a>] panic+0x1e3/0x3a1
 [<ffffffff815051a7>] ? set_ti_thread_flag+0x1e/0x1e
 [<ffffffff81505a58>] ? rt_mutex_top_waiter.part.10+0x6/0x6
 [<ffffffff83a4af46>] ? p9_idpool_put+0x36/0x40
 [<ffffffff816c85d2>] ? drop_nlink+0xc2/0x120
 [<ffffffff8126df4f>] __warn+0x1bf/0x1e0
 [<ffffffff8126e13c>] warn_slowpath_null+0x2c/0x40
 [<ffffffff816c85d2>] drop_nlink+0xc2/0x120
 [<ffffffff81b17106>] v9fs_remove+0x256/0x2d0
 [<ffffffff81b1719f>] v9fs_vfs_unlink+0x1f/0x30
 [<ffffffff816a7130>] vfs_unlink+0x2b0/0x490
 [<ffffffff816a7840>] do_unlinkat+0x530/0x660
 [<ffffffff816a7310>] ? vfs_unlink+0x490/0x490
 [<ffffffff81002f2f>] ? syscall_trace_enter+0x3cf/0xdb0
 [<ffffffff8150465a>] ? __context_tracking_exit.part.4+0x9a/0x1e0
 [<ffffffff81002b60>] ? exit_to_usermode_loop+0x190/0x190
 [<ffffffff82001b07>] ? check_preemption_disabled+0x37/0x1e0
 [<ffffffff82001cec>] ? __this_cpu_preempt_check+0x1c/0x20
 [<ffffffff816a79f0>] ? SyS_unlinkat+0x80/0x80
 [<ffffffff816a7a0a>] SyS_unlink+0x1a/0x20
 [<ffffffff81005524>] do_syscall_64+0x1c4/0x4e0
 [<ffffffff83c3286a>] entry_SYSCALL64_slow_path+0x25/0x25

It seems to happen when fstat() calls into v9fs_vfs_getattr_dotl() and
that does a v9fs_stat2inode_dotl() which changes inode->i_nlink from 1
to 0 (without any locking). The unlink() which was already in progress
then sees 0 when it does drop_nlink().

My mount flags are:

  trans=virtio,version=9p2000.L

If I add "cache=loose" then there is no warning (as expected, since
v9fs_vfs_getattr_dotl() bails out early in that case and doesn't change
the in-memory inode).


Vegard