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SubjectRe: [bug, 2.6.26-rc4/rc5] sporadic bootup crashes in blk_lookup_devt()/prepare_namespace()
On 6/9/08, Vegard Nossum <vegard.nossum@gmail.com> wrote:
> It seems that this list (block_class.devices) is protected by
> block_class_lock in block/genhd.c. This list is only ever modified by
> device_add() and device_del() in drivers/base/core.c. Both of those
> are (only) protected by dev->class->sem, however. Is there a locking
> mismatch here? But none of the locking code here seems to be changed
> in years...

I think this seems correct.

Everywhere else where we traverse the struct class->devices list, they
have down(&class->sem); first and up(&class->sem); afterwards.

Commit fd04897bb20be29d60f7e426a053545aebeaa61a even has this hunk:
@@ -177,8 +177,7 @@ struct class {
struct list_head devices;
struct list_head interfaces;
struct kset class_dirs;
- struct semaphore sem; /* locks both the children and interface
-
+ struct semaphore sem; /* locks children, devices, interfaces */
struct class_attribute * class_attrs;
struct class_device_attribute * class_dev_attrs;
struct device_attribute * dev_attrs;

So why doesn't block/genhd.c do this too? It seems to me that the
mutex locking here is simply a remnant of old code that happened to
not crash in most cases by chance.


Vegard

--
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
-- E. W. Dijkstra, EWD1036


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