Re: [PATCH] Yama: remove locking from delete path

[Kees Cook]

On Tue, Nov 20, 2012 at 4:24 AM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> Kees Cook wrote:
>> Instead of locking the list during a delete, mark entries as invalid
>> and trigger a workqueue to clean them up. This lets us easily handle
>> task_free from interrupt context.
>
>> @@ -57,9 +80,12 @@ static int yama_ptracer_add(struct task_struct *tracer,
>>
>>       added->tracee = tracee;
>>       added->tracer = tracer;
>> +     added->invalid = false;
>>
>> -     spin_lock_bh(&ptracer_relations_lock);
>> +     spin_lock(&ptracer_relations_lock);
>
> Can't you use
>         spin_lock_irqsave(&ptracer_relations_lock, flags);
>         spin_unlock_irqrestore(&ptracer_relations_lock, flags);
> instead of adding ->invalid ?

The _bh was sufficient originally, but looking at Sasha's deadlock, it
seems like I should get rid of locking entirely on this path. What do
you think of this report:
https://lkml.org/lkml/2012/10/17/600

I'm concerned that blocking interrupts would be an even more expensive
solution, since every task_free() is forced to block interrupts
briefly. Most systems will have either an empty relations list, or a
very short one, so it seemed better to avoid any locking at all on the
task_free() path. Now the locking contention would be moved to being
between the workqueue and any add calls.

-Kees

-Kees

-- 
Kees Cook
Chrome OS Security
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