On 2016-08-31 03:54 AM, Catalin Marinas wrote:
> On Tue, Aug 30, 2016 at 02:35:12PM -0400, Nicholas Krause wrote:
>> This fixes a issue in the current locking logic of the function,
>> __delete_object where we are trying to attempt to lock the passed
>> object structure's spinlock again after being previously held
>> elsewhere by the kmemleak code. Fix this by instead of assuming
>> we are the only one contending for the object's lock their are
>> possible other users and create two branches, one where we get
>> the lock when calling spin_trylock_irqsave on the object's lock
>> and the other when the lock is held else where by kmemleak.
>
> Have you actually got a deadlock that requires this fix?
>
Yes I have got a deadlock that this does fix.
Nick
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -631,12 +631,19 @@ static void __delete_object(struct kmemleak_object *object)
>>
>> /*
>> * Locking here also ensures that the corresponding memory block
>> - * cannot be freed when it is being scanned.
>> + * cannot be freed when it is being scanned. Further more the
>> + * object's lock may have been previously holded by another holder
>> + * in the kmemleak code, therefore attempt to lock the object's lock
>> + * before holding it and unlocking it.
>> */
>> - spin_lock_irqsave(&object->lock, flags);
>> - object->flags &= ~OBJECT_ALLOCATED;
>> - spin_unlock_irqrestore(&object->lock, flags);
>> - put_object(object);
>> + if (spin_trylock_irqsave(&object->lock, flags)) {
>> + object->flags &= ~OBJECT_ALLOCATED;
>> + spin_unlock_irqrestore(&object->lock, flags);
>> + put_object(object);
>> + } else {
>> + object->flags &= ~OBJECT_ALLOCATED;
>> + put_object(object);
>> + }
>
> NAK. This lock here is needed, as described in the comment, to prevent
> an object being freed while it is being scanned. The scan_object()
> function acquires the same lock and checks for OBJECT_ALLOCATED before
> accessing the memory (which could be vmalloc'ed for example, so freeing
> would cause a page fault).
>
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