[PATCH 03/11] target: Avoid that EXTENDED COPY commands trigger lock inversion

[Bart Van Assche <bart.vanassche@wdc.com>]

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@wdc.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Mike Christie <mchristi@redhat.com>
---
 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