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block: fix block_class iteration locking

PATCH In review Tejun Heo 2 participants 4 messages Jul 14, 2008 – Jul 21, 2008 Block layer

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Tejun Heo

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#0 lore raw

PATCH 1/1 block: fix block_class iteration locking

genhd.c used block_class_lock when it wants to iterate over block_class.devices list. This is incorrect as block_class_lock has nothing to do with synchronization around block_class.devices list, so there's a window, albeit small, where the list can change while it's being iterated over. Fix it by using block_class.sem instead.

Signed-off-by Tejun Heo <[email protected]>
---
 block/genhd.c |   16 ++++++++--------
 1 files changed, 8 insertions(+), 8 deletions(-)
block/genhd.c +8 -8
index b922d48..b646e7e 100644
@@ -234,7 +234,7 @@ void __init printk_all_partitions(void)
 	char buf[BDEVNAME_SIZE];
 	int n;
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	/* For each block device... */
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)

@@ -275,7 +275,7 @@ void __init printk_all_partitions(void)
 		}
 	}
 
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 }
 
 #ifdef CONFIG_PROC_FS

@@ -285,7 +285,7 @@ static void *part_start(struct seq_file *part, loff_t *pos)
 	loff_t k = *pos;
 	struct device *dev;
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)
 			continue;

@@ -311,7 +311,7 @@ static void *part_next(struct seq_file *part, void *v, loff_t *pos)
 
 static void part_stop(struct seq_file *part, void *v)
 {
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 }
 
 static int show_partition(struct seq_file *part, void *v)

@@ -533,7 +533,7 @@ static void *diskstats_start(struct seq_file *part, loff_t *pos)
 	loff_t k = *pos;
 	struct device *dev;
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)
 			continue;

@@ -560,7 +560,7 @@ static void *diskstats_next(struct seq_file *part, void *v, loff_t *pos)
 
 static void diskstats_stop(struct seq_file *part, void *v)
 {
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 }
 
 static int diskstats_show(struct seq_file *s, void *v)

@@ -658,7 +658,7 @@ dev_t blk_lookup_devt(const char *name, int part)
 	struct device *dev;
 	dev_t devt = MKDEV(0, 0);
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)
 			continue;

@@ -671,7 +671,7 @@ dev_t blk_lookup_devt(const char *name, int part)
 			break;
 		}
 	}
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 
 	return devt;
 }
Signature · 1 line
1.5.4.5
#1 lore raw
Quoted 7 lines · Tejun Heo

genhd.c used block_class_lock when it wants to iterate over block_class.devices list. This is incorrect as block_class_lock has nothing to do with synchronization around block_class.devices list, so there's a window, albeit small, where the list can change while it's being iterated over. Fix it by using block_class.sem instead.

Signed-off-by Tejun Heo <[email protected]>

Greg has some patches sitting in his tree that convert those functions to use class_{for_each,find}_device() instead - and that looks like the clean solution, especially since the class implementation details will be moved into a private structure. One worry though...

Quoted 28 lines
block/genhd.c +3 -3
@@ -285,7 +285,7 @@ static void *part_start(struct seq_file *part, loff_t *pos)
 	loff_t k = *pos;
 	struct device *dev;
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)
 			continue;

@@ -311,7 +311,7 @@ static void *part_next(struct seq_file *part, void *v, loff_t *pos)
 
 static void part_stop(struct seq_file *part, void *v)
 {
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 }
 
 static int show_partition(struct seq_file *part, void *v)

@@ -533,7 +533,7 @@ static void *diskstats_start(struct seq_file *part, loff_t *pos)
 	loff_t k = *pos;
 	struct device *dev;
 
-	mutex_lock(&block_class_lock);
+	down(&block_class.sem);
 	list_for_each_entry(dev, &block_class.devices, node) {
 		if (dev->type != &disk_type)
 			continue;

block/genhd.c static void *diskstats_next(struct seq_file *part, void *v, loff_t *pos) · line 566

Quoted 8 lines
block/genhd.c +1 -1
@@ -560,7 +560,7 @@ static void *diskstats_next(struct seq_file *part, void *v, loff_t *pos)
 
 static void diskstats_stop(struct seq_file *part, void *v)
 {
-	mutex_unlock(&block_class_lock);
+	up(&block_class.sem);
 }
 
 static int diskstats_show(struct seq_file *s, void *v)

With your patch, you protect against adding/removing devices to/from the class between _start and _stop. This wasn't done previously, and won't be done with Greg's patches (which leaves in the locking/unlocking of block_class_lock). Not sure if I'm missing something here...

author
#2 lore raw
Quoted 4 lines · Cornelia Huck

Greg has some patches sitting in his tree that convert those functions to use class_{for_each,find}_device() instead - and that looks like the clean solution, especially since the class implementation details will be moved into a private structure. One worry though...

Yeah, that sounds nice.

Quoted 5 lines

With your patch, you protect against adding/removing devices to/from the class between _start and _stop. This wasn't done previously, and won't be done with Greg's patches (which leaves in the locking/unlocking of block_class_lock). Not sure if I'm missing something here...

That's mostly the point of this patch. It wasn't protected properly before. It would be best if there's a safe iterator (maybe use klist?).

Thanks.

Signature · 1 line
tejun
#3 lore raw
Quoted 8 lines · Tejun Heo
Quoted 5 lines

With your patch, you protect against adding/removing devices to/from the class between _start and _stop. This wasn't done previously, and won't be done with Greg's patches (which leaves in the locking/unlocking of block_class_lock). Not sure if I'm missing something here...

That's mostly the point of this patch. It wasn't protected properly before. It would be best if there's a safe iterator (maybe use klist?).

Hm, I don't see how a klist would help here since it still allows to add/remove devices during walking the file.

What we need is more like a "lock/unlock the device list" operation and a lockless iterator - that would give the same semantics as your patch.