Messages in this thread | | | Date | Tue, 8 Dec 2009 19:37:58 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC,PATCH 14/14] utrace core |
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On 12/08, Peter Zijlstra wrote: > > 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?
Yes. That is why utrace_control() (which does unlock) should take the ref, not utrace_reset().
> > 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?
We can, but then ->release() will be called in atomic context. Utrace tries hard to not "restrict" the module writers.
> But yeah, passing that list along does seem like a better solution.
Well, it has multiple callers, everyone will be complicated.
Oleg.
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