On Tue, 2009-12-08 at 17:31 +0100, Oleg Nesterov wrote: > > If you take a task ref you can write the much saner: > > > > utrace_control() > > { > > ... > > spin_lock(&utrace->lock); > > ... > > if (reset) > > utrace_reset(utrace); > > > > spin_unlock(&utrace->lock); > > } > > No, get_task_struct() in utrace_reset() can't help, we should move > it into utrace_control() then. And in this case it becomes even more > subtle: it is needed because ->utrace_flags may be cleared inside > utrace_reset() and after that utrace_control()->spin_unlock() becomes > unsafe.
The task->utrace pointer is cleaned up on free_task()->tracehook_free_task()->utrace_free_task(), so by holding a ref on the task, we ensure ->utrace stays around, and we can do spin_unlock(), right? > Also. utrace_reset() drops utrace->lock to call put_detached_list() > lockless. If we want to avoid the assymetric locking, every caller > should pass "struct list_head *detached" to utrace_reset(), drop > utrace->lock, and call put_detached_list(). All that seems to do is call ->release() and kmem_cache_free()s the utrace_engine thing, why can't that be done with utrace->lock held? But yeah, passing that list along does seem like a better solution.