[PATCH 3.16 097/328] scsi: core: Avoid that SCSI device removal through sysfs triggers a deadlock

Ben Hutchings
2018-12-09T22:26:15Z

From: Bart Van Assche <bart.vanassche@wdc.com>

commit 0ee223b2e1f67cb2de9c0e3247c510d846e74d63 upstream.

A long time ago the unfortunate decision was taken to add a self-deletion
attribute to the sysfs SCSI device directory. That decision was unfortunate
because self-deletion is really tricky. We can't drop that attribute
because widely used user space software depends on it, namely the
rescan-scsi-bus.sh script. Hence this patch that avoids that writing into
that attribute triggers a deadlock. See also commit 7973cbd9fbd9 ("[PATCH]
add sysfs attributes to scan and delete scsi_devices").

This patch avoids that self-removal triggers the following deadlock:

======================================================
WARNING: possible circular locking dependency detected
4.18.0-rc2-dbg+ #5 Not tainted
------------------------------------------------------
modprobe/6539 is trying to acquire lock:
000000008323c4cd (kn->count#202){++++}, at: kernfs_remove_by_name_ns+0x45/0x90

but task is already holding lock:
00000000a6ec2c69 (&shost->scan_mutex){+.+.}, at: scsi_remove_host+0x21/0x150 [scsi_mod]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&shost->scan_mutex){+.+.}:
__mutex_lock+0xfe/0xc70
mutex_lock_nested+0x1b/0x20
scsi_remove_device+0x26/0x40 [scsi_mod]
sdev_store_delete+0x27/0x30 [scsi_mod]
dev_attr_store+0x3e/0x50
sysfs_kf_write+0x87/0xa0
kernfs_fop_write+0x190/0x230
__vfs_write+0xd2/0x3b0
vfs_write+0x101/0x270
ksys_write+0xab/0x120
__x64_sys_write+0x43/0x50
do_syscall_64+0x77/0x230
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (kn->count#202){++++}:
lock_acquire+0xd2/0x260
__kernfs_remove+0x424/0x4a0
kernfs_remove_by_name_ns+0x45/0x90
remove_files.isra.1+0x3a/0x90
sysfs_remove_group+0x5c/0xc0
sysfs_remove_groups+0x39/0x60
device_remove_attrs+0x82/0xb0
device_del+0x251/0x580
__scsi_remove_device+0x19f/0x1d0 [scsi_mod]
scsi_forget_host+0x37/0xb0 [scsi_mod]
scsi_remove_host+0x9b/0x150 [scsi_mod]
sdebug_driver_remove+0x4b/0x150 [scsi_debug]
device_release_driver_internal+0x241/0x360
device_release_driver+0x12/0x20
bus_remove_device+0x1bc/0x290
device_del+0x259/0x580
device_unregister+0x1a/0x70
sdebug_remove_adapter+0x8b/0xf0 [scsi_debug]
scsi_debug_exit+0x76/0xe8 [scsi_debug]
__x64_sys_delete_module+0x1c1/0x280
do_syscall_64+0x77/0x230
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&shost->scan_mutex);
lock(kn->count#202);
lock(&shost->scan_mutex);
lock(kn->count#202);

*** DEADLOCK ***

2 locks held by modprobe/6539:
#0: 00000000efaf9298 (&dev->mutex){....}, at: device_release_driver_internal+0x68/0x360
#1: 00000000a6ec2c69 (&shost->scan_mutex){+.+.}, at: scsi_remove_host+0x21/0x150 [scsi_mod]

stack backtrace:
CPU: 10 PID: 6539 Comm: modprobe Not tainted 4.18.0-rc2-dbg+ #5
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
dump_stack+0xa4/0xf5
print_circular_bug.isra.34+0x213/0x221
__lock_acquire+0x1a7e/0x1b50
lock_acquire+0xd2/0x260
__kernfs_remove+0x424/0x4a0
kernfs_remove_by_name_ns+0x45/0x90
remove_files.isra.1+0x3a/0x90
sysfs_remove_group+0x5c/0xc0
sysfs_remove_groups+0x39/0x60
device_remove_attrs+0x82/0xb0
device_del+0x251/0x580
__scsi_remove_device+0x19f/0x1d0 [scsi_mod]
scsi_forget_host+0x37/0xb0 [scsi_mod]
scsi_remove_host+0x9b/0x150 [scsi_mod]
sdebug_driver_remove+0x4b/0x150 [scsi_debug]
device_release_driver_internal+0x241/0x360
device_release_driver+0x12/0x20
bus_remove_device+0x1bc/0x290
device_del+0x259/0x580
device_unregister+0x1a/0x70
sdebug_remove_adapter+0x8b/0xf0 [scsi_debug]
scsi_debug_exit+0x76/0xe8 [scsi_debug]
__x64_sys_delete_module+0x1c1/0x280
do_syscall_64+0x77/0x230
entry_SYSCALL_64_after_hwframe+0x49/0xbe

See also https://www.mail-archive.com/linux-scsi@vger.kernel.org/msg54525.html.

Fixes: ac0ece9174ac ("scsi: use device_remove_file_self() instead of device_schedule_callback()")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Thumshirn <jthumshirn@suse.de>

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/scsi/scsi_sysfs.c | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)

--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -651,8 +651,24 @@ static ssize_t
sdev_store_delete(struct device *dev, struct device_attribute *attr,
const char *buf, size_t count)
{
- if (device_remove_file_self(dev, attr))
- scsi_remove_device(to_scsi_device(dev));
+ struct kernfs_node *kn;
+
+ kn = sysfs_break_active_protection(&dev->kobj, &attr->attr);
+ WARN_ON_ONCE(!kn);
+ /*
+ * Concurrent writes into the "delete" sysfs attribute may trigger
+ * concurrent calls to device_remove_file() and scsi_remove_device().
+ * device_remove_file() handles concurrent removal calls by
+ * serializing these and by ignoring the second and later removal
+ * attempts. Concurrent calls of scsi_remove_device() are
+ * serialized. The second and later calls of scsi_remove_device() are
+ * ignored because the first call of that function changes the device
+ * state into SDEV_DEL.
+ */
+ device_remove_file(dev, attr);
+ scsi_remove_device(to_scsi_device(dev));
+ if (kn)
+ sysfs_unbreak_active_protection(kn);
return count;
};
static DEVICE_ATTR(delete, S_IWUSR, NULL, sdev_store_delete);