The lockdep code had reported the following unsafe locking scenario:
CPU0 CPU1
---- ----
lock(s_active#228);
lock(&bdev->bd_mutex/1);
lock(s_active#228);
lock(&bdev->bd_mutex);
*** DEADLOCK ***
The deadlock may happen when one task (CPU1) is trying to delete
a partition in a block device and another task (CPU0) is accessing
tracing sysfs file in that partition.
To avoid that, accessing tracing sysfs file will now use a mutex
trylock loop and the operation will fail if a delete operation is
in progress.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
block/ioctl.c | 3 +++
include/linux/fs.h | 1 +
kernel/trace/blktrace.c | 24 ++++++++++++++++++++++--
3 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/block/ioctl.c b/block/ioctl.c
index 0de02ee..7211608 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -86,12 +86,15 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
return -EBUSY;
}
/* all seems OK */
+ bdev->bd_deleting = 1;
fsync_bdev(bdevp);
invalidate_bdev(bdevp);
mutex_lock_nested(&bdev->bd_mutex, 1);
delete_partition(disk, partno);
mutex_unlock(&bdev->bd_mutex);
+ bdev->bd_deleting = 0;
+
mutex_unlock(&bdevp->bd_mutex);
bdput(bdevp);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7b5d681..5d4ae9d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -427,6 +427,7 @@ struct block_device {
#endif
struct block_device * bd_contains;
unsigned bd_block_size;
+ int bd_deleting;
struct hd_struct * bd_part;
/* number of times partitions within this device have been opened. */
unsigned bd_part_count;
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index bc364f8..715e77e 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1605,6 +1605,18 @@ static struct request_queue *blk_trace_get_queue(struct block_device *bdev)
return bdev_get_queue(bdev);
}
+/*
+ * Read/write to the tracing sysfs file requires taking references to the
+ * sysfs file and then acquiring the bd_mutex. Deleting a block device
+ * requires acquiring the bd_mutex and then waiting for all the sysfs
+ * references to be gone. This can lead to deadlock if both operations
+ * happen simultaneously. To avoid this problem, read/write to the
+ * the tracing sysfs files can now fail if the bd_mutex cannot be
+ * acquired while a deletion operation is in progress.
+ *
+ * A mutex trylock loop is used assuming that tracing sysfs operations
+ * aren't frequently enough to cause any contention problem.
+ */
static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
struct device_attribute *attr,
char *buf)
@@ -1622,7 +1634,11 @@ static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
if (q == NULL)
goto out_bdput;
- mutex_lock(&bdev->bd_mutex);
+ while (!mutex_trylock(&bdev->bd_mutex)) {
+ if (bdev->bd_deleting)
+ goto out_bdput;
+ schedule();
+ }
if (attr == &dev_attr_enable) {
ret = sprintf(buf, "%u\n", !!q->blk_trace);
@@ -1683,7 +1699,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
if (q == NULL)
goto out_bdput;
- mutex_lock(&bdev->bd_mutex);
+ while (!mutex_trylock(&bdev->bd_mutex)) {
+ if (bdev->bd_deleting)
+ goto out_bdput;
+ schedule();
+ }
if (attr == &dev_attr_enable) {
if (value)
--
1.8.3.1
