The approach for adding a device to the devices_idr data structure
and for removing it is as follows:
* &dev->dev_group.cg_item is initialized before a device is added
to devices_idr.
* If the reference count of a device drops to zero then
target_free_device() removes the device from devices_idr.
* All devices_idr manipulations are protected by device_mutex.
This means that increasing the reference count of a device is
sufficient to prevent removal from devices_idr and also that it is
safe access dev_group.cg_item for any device that is referenced by
devices_idr. Use this to modify target_find_device() and
target_for_each_device() such that these functions no longer
introduce a dependency between device_mutex and the configfs root
inode mutex.
Note: it is safe to pass a NULL pointer to config_item_put() and
also to config_item_get_unless_zero().
This patch prevents that lockdep reports the following complaint:
======================================================
WARNING: possible circular locking dependency detected
4.12.0-rc1-dbg+ #1 Not tainted
------------------------------------------------------
rmdir/12053 is trying to acquire lock:
(device_mutex#2){+.+.+.}, at: [<ffffffffa010afce>] target_free_device+0xae/0xf0 [target_core_mod]
but task is already holding lock:
(&sb->s_type->i_mutex_key#14){++++++}, at: [<ffffffff811c5c30>] vfs_rmdir+0x50/0x140
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&sb->s_type->i_mutex_key#14){++++++}:
lock_acquire+0x59/0x80
down_write+0x36/0x70
configfs_depend_item+0x3a/0xb0 [configfs]
target_depend_item+0x13/0x20 [target_core_mod]
target_xcopy_locate_se_dev_e4_iter+0x87/0x100 [target_core_mod]
target_devices_idr_iter+0x16/0x20 [target_core_mod]
idr_for_each+0x39/0xc0
target_for_each_device+0x36/0x50 [target_core_mod]
target_xcopy_locate_se_dev_e4+0x28/0x80 [target_core_mod]
target_xcopy_do_work+0x2e9/0xdd0 [target_core_mod]
process_one_work+0x1ca/0x3f0
worker_thread+0x49/0x3b0
kthread+0x109/0x140
ret_from_fork+0x31/0x40
-> #0 (device_mutex#2){+.+.+.}:
__lock_acquire+0x101f/0x11d0
lock_acquire+0x59/0x80
__mutex_lock+0x7e/0x950
mutex_lock_nested+0x16/0x20
target_free_device+0xae/0xf0 [target_core_mod]
target_core_dev_release+0x10/0x20 [target_core_mod]
config_item_put+0x6e/0xb0 [configfs]
configfs_rmdir+0x1a6/0x300 [configfs]
vfs_rmdir+0xb7/0x140
do_rmdir+0x1f4/0x200
SyS_rmdir+0x11/0x20
entry_SYSCALL_64_fastpath+0x23/0xc2
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&sb->s_type->i_mutex_key#14);
lock(device_mutex#2);
lock(&sb->s_type->i_mutex_key#14);
lock(device_mutex#2);
*** DEADLOCK ***
3 locks held by rmdir/12053:
#0: (sb_writers#10){.+.+.+}, at: [<ffffffff811e223f>] mnt_want_write+0x1f/0x50
#1: (&sb->s_type->i_mutex_key#14/1){+.+.+.}, at: [<ffffffff811cb97e>] do_rmdir+0x15e/0x200
#2: (&sb->s_type->i_mutex_key#14){++++++}, at: [<ffffffff811c5c30>] vfs_rmdir+0x50/0x140
stack backtrace:
CPU: 3 PID: 12053 Comm: rmdir Not tainted 4.12.0-rc1-dbg+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
dump_stack+0x86/0xcf
print_circular_bug+0x1c7/0x220
__lock_acquire+0x101f/0x11d0
lock_acquire+0x59/0x80
__mutex_lock+0x7e/0x950
mutex_lock_nested+0x16/0x20
target_free_device+0xae/0xf0 [target_core_mod]
target_core_dev_release+0x10/0x20 [target_core_mod]
config_item_put+0x6e/0xb0 [configfs]
configfs_rmdir+0x1a6/0x300 [configfs]
vfs_rmdir+0xb7/0x140
do_rmdir+0x1f4/0x200
SyS_rmdir+0x11/0x20
entry_SYSCALL_64_fastpath+0x23/0xc2
Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx>
Cc: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Mike Christie <mchristi@xxxxxxxxxx>
---
drivers/target/target_core_device.c | 31 ++++++++++++++++++++++++-------
1 file changed, 24 insertions(+), 7 deletions(-)
diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
index e8dd6da164b2..c7535907e2d1 100644
--- a/drivers/target/target_core_device.c
+++ b/drivers/target/target_core_device.c
@@ -893,25 +893,36 @@ EXPORT_SYMBOL(target_to_linux_sector);
struct se_device *target_find_device(int id, bool do_depend)
{
struct se_device *dev;
+ struct config_item *item = NULL;
mutex_lock(&device_mutex);
dev = idr_find(&devices_idr, id);
- if (dev && do_depend && target_depend_item(&dev->dev_group.cg_item))
- dev = NULL;
+ if (dev)
+ item = config_item_get_unless_zero(&dev->dev_group.cg_item);
mutex_unlock(&device_mutex);
+
+ if (dev && (!item || (do_depend && target_depend_item(item) < 0)))
+ dev = NULL;
+ config_item_put(item);
return dev;
}
EXPORT_SYMBOL(target_find_device);
struct devices_idr_iter {
+ struct config_item *prev_item;
int (*fn)(struct se_device *dev, void *data);
void *data;
};
static int target_devices_idr_iter(int id, void *p, void *data)
+ __must_hold(&device_mutex)
{
struct devices_idr_iter *iter = data;
struct se_device *dev = p;
+ int ret;
+
+ config_item_put(iter->prev_item);
+ iter->prev_item = NULL;
/*
* We add the device early to the idr, so it can be used
@@ -922,7 +933,15 @@ static int target_devices_idr_iter(int id, void *p, void *data)
if (!(dev->dev_flags & DF_CONFIGURED))
return 0;
- return iter->fn(dev, iter->data);
+ iter->prev_item = config_item_get_unless_zero(&dev->dev_group.cg_item);
+ if (!iter->prev_item)
+ return 0;
+ mutex_unlock(&device_mutex);
+
+ ret = iter->fn(dev, iter->data);
+
+ mutex_lock(&device_mutex);
+ return ret;
}
/**
@@ -936,15 +955,13 @@ static int target_devices_idr_iter(int id, void *p, void *data)
int target_for_each_device(int (*fn)(struct se_device *dev, void *data),
void *data)
{
- struct devices_idr_iter iter;
+ struct devices_idr_iter iter = { .fn = fn, .data = data };
int ret;
- iter.fn = fn;
- iter.data = data;
-
mutex_lock(&device_mutex);
ret = idr_for_each(&devices_idr, target_devices_idr_iter, &iter);
mutex_unlock(&device_mutex);
+ config_item_put(iter.prev_item);
return ret;
}
--
2.15.1
--
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