Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

Ming Lei
2021-10-20T00:56:33Z

On Tue, Oct 19, 2021 at 12:38:42PM -0700, Luis Chamberlain wrote:
> On Wed, Oct 20, 2021 at 12:39:22AM +0800, Ming Lei wrote:
> > On Tue, Oct 19, 2021 at 08:50:24AM -0700, Luis Chamberlain wrote:
> > > So do you want to take the position:
> > >
> > > Hey driver authors: you cannot use any shared lock on module removal and
> > > on sysfs ops?
> >
> > IMO, yes, in your patch of 'zram: fix crashes with cpu hotplug multistate',
> > when you added mutex_lock(zram_index_mutex) to disksize_store() and
> > other attribute show() or store() method. You have added new deadlock
> > between hot_remove_store() and disksize_store() & others, which can't be
> > addressed by your approach of holding module refcnt.
> >
> > So far not see ltp tests covers hot add/remove interface yet.
>
> Care to show what commands to use to cause this deadlock with my patches?

Build a kernel with your patch 4,7,8,9,11 and 12(all others are test module or
document change), with lockdep enabled, run the following command, then you
will see the warning, and it is one real deadlock, not false warning.

BTW, your patch 9 can't be applied cleanly against both linus and next
tree, so I edited it manually, but that can't make difference wrt. this issue.

[root@ktest-09 ~]# lsblk | grep zram
zram0 253:0 0 0B 0 disk
cat /sys/class/zram-control/hot_add
[root@ktest-09 ~]# lsblk | grep zram
zram0 253:0 0 0B 0 disk
zram1 253:1 0 0B 0 disk
[root@ktest-09 ~]# echo 256M > /sys/block/zram1/disksize
[root@ktest-09 ~]# echo 1 > /sys/class/zram-control/hot_remove
[root@ktest-09 ~]# dmesg
...
[ 75.599882] ======================================================
[ 75.601355] WARNING: possible circular locking dependency detected
[ 75.602818] 5.15.0-rc3_zram_fix_luis+ #24 Not tainted
[ 75.604038] ------------------------------------------------------
[ 75.605512] bash/1154 is trying to acquire lock:
[ 75.606634] ffff91ce026cd428 (kn->active#237){++++}-{0:0}, at: __kernfs_remove+0x1ab/0x1e0
[ 75.608570]
but task is already holding lock:
[ 75.609955] ffffffff839e3ef0 (zram_index_mutex){+.+.}-{3:3}, at: hot_remove_store+0x52/0xf0
[ 75.611910]
which lock already depends on the new lock.

[ 75.613896]
the existing dependency chain (in reverse order) is:
[ 75.615830]
-> #1 (zram_index_mutex){+.+.}-{3:3}:
[ 75.617483] __lock_acquire+0x4d2/0x930
[ 75.618650] lock_acquire+0xbb/0x2d0
[ 75.619748] __mutex_lock+0x8e/0x8a0
[ 75.620854] disksize_store+0x38/0x180
[ 75.621996] kernfs_fop_write_iter+0x134/0x1d0
[ 75.623287] new_sync_write+0x122/0x1b0
[ 75.624442] vfs_write+0x23e/0x350
[ 75.625506] ksys_write+0x68/0xe0
[ 75.626550] do_syscall_64+0x3b/0x90
[ 75.627649] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 75.629070]
-> #0 (kn->active#237){++++}-{0:0}:
[ 75.630677] check_prev_add+0x91/0xc10
[ 75.631816] validate_chain+0x474/0x500
[ 75.632972] __lock_acquire+0x4d2/0x930
[ 75.634131] lock_acquire+0xbb/0x2d0
[ 75.635234] kernfs_drain+0x139/0x190
[ 75.636355] __kernfs_remove+0x1ab/0x1e0
[ 75.637532] kernfs_remove_by_name_ns+0x3f/0x80
[ 75.638843] remove_files+0x2b/0x60
[ 75.639926] sysfs_remove_group+0x38/0x80
[ 75.641120] sysfs_remove_groups+0x29/0x40
[ 75.642334] device_remove_attrs+0x5b/0x90
[ 75.643552] device_del+0x184/0x400
[ 75.644635] zram_remove+0xac/0xc0
[ 75.645700] hot_remove_store+0xa3/0xf0
[ 75.646856] kernfs_fop_write_iter+0x134/0x1d0
[ 75.648147] new_sync_write+0x122/0x1b0
[ 75.649311] vfs_write+0x23e/0x350
[ 75.650372] ksys_write+0x68/0xe0
[ 75.651412] do_syscall_64+0x3b/0x90
[ 75.652512] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 75.653929]
other info that might help us debug this:

[ 75.656054] Possible unsafe locking scenario:

[ 75.657637] CPU0 CPU1
[ 75.658833] ---- ----
[ 75.660020] lock(zram_index_mutex);
[ 75.661024] lock(kn->active#237);
[ 75.662549] lock(zram_index_mutex);
[ 75.664103] lock(kn->active#237);
[ 75.665072]
*** DEADLOCK ***

[ 75.666736] 4 locks held by bash/1154:
[ 75.667767] #0: ffff91ce06983470 (sb_writers#4){.+.+}-{0:0}, at: ksys_write+0x68/0xe0
[ 75.669802] #1: ffff91ce4123d290 (&of->mutex){+.+.}-{3:3}, at: kernfs_fop_write_iter+0x100/0x1d0
[ 75.672050] #2: ffff91ce05a7ac40 (kn->active#238){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x108/0x1d0
[ 75.674383] #3: ffffffff839e3ef0 (zram_index_mutex){+.+.}-{3:3}, at: hot_remove_store+0x52/0xf0
[ 75.676595]
stack backtrace:
[ 75.677835] CPU: 2 PID: 1154 Comm: bash Not tainted 5.15.0-rc3_zram_fix_luis+ #24
[ 75.679768] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1.fc33 04/01/2014
[ 75.681927] Call Trace:
[ 75.682674] dump_stack_lvl+0x57/0x7d
[ 75.683680] check_noncircular+0xff/0x110
[ 75.684758] ? stack_trace_save+0x4b/0x70
[ 75.685843] check_prev_add+0x91/0xc10
[ 75.686867] ? add_chain_cache+0x112/0x2d0
[ 75.687965] validate_chain+0x474/0x500
[ 75.689005] __lock_acquire+0x4d2/0x930
[ 75.690054] lock_acquire+0xbb/0x2d0
[ 75.691038] ? __kernfs_remove+0x1ab/0x1e0
[ 75.692131] ? __lock_release+0x179/0x2c0
[ 75.693212] ? kernfs_drain+0x5b/0x190
[ 75.694239] kernfs_drain+0x139/0x190
[ 75.695240] ? __kernfs_remove+0x1ab/0x1e0
[ 75.696341] __kernfs_remove+0x1ab/0x1e0
[ 75.697408] kernfs_remove_by_name_ns+0x3f/0x80
[ 75.698607] remove_files+0x2b/0x60
[ 75.699576] sysfs_remove_group+0x38/0x80
[ 75.700661] sysfs_remove_groups+0x29/0x40
[ 75.701770] device_remove_attrs+0x5b/0x90
[ 75.702870] device_del+0x184/0x400
[ 75.703835] zram_remove+0xac/0xc0
[ 75.704785] hot_remove_store+0xa3/0xf0
[ 75.705831] kernfs_fop_write_iter+0x134/0x1d0
[ 75.707004] new_sync_write+0x122/0x1b0
[ 75.708048] ? __do_fast_syscall_32+0xe0/0xf0
[ 75.709214] vfs_write+0x23e/0x350
[ 75.710161] ksys_write+0x68/0xe0
[ 75.711088] do_syscall_64+0x3b/0x90
[ 75.712078] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 75.713389] RIP: 0033:0x7fcc1893f927
[ 75.714381] Code: 0f 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
[ 75.718879] RSP: 002b:00007ffcd56d91a8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 75.720832] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007fcc1893f927
[ 75.722592] RDX: 0000000000000002 RSI: 000055d7d33f78c0 RDI: 0000000000000001
[ 75.724352] RBP: 000055d7d33f78c0 R08: 0000000000000000 R09: 00007fcc189f44e0
[ 75.726123] R10: 00007fcc189f43e0 R11: 0000000000000246 R12: 0000000000000002
[ 75.727884] R13: 00007fcc18a395a0 R14: 0000000000000002 R15: 00007fcc18a397a0

Thanks,
Ming