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SubjectRe: [PATCH] sigqueue_free: fix the race with collect_signal()
Oleg Nesterov wrote:
> On 08/24, taoyue wrote:
>
>> Oleg Nesterov wrote:
>>
>>> --- t/kernel/signal.c~SQFREE 2007-08-22 20:06:31.000000000 +0400
>>> +++ t/kernel/signal.c 2007-08-23 16:02:57.000000000 +0400
>>> @@ -1297,20 +1297,19 @@ struct sigqueue *sigqueue_alloc(void)
>>> void sigqueue_free(struct sigqueue *q)
>>> {
>>> unsigned long flags;
>>> + spinlock_t *lock = &current->sighand->siglock;
>>> +
>>> BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
>>> /*
>>> * If the signal is still pending remove it from the
>>> - * pending queue.
>>> + * pending queue. We must hold ->siglock while testing
>>> + * q->list to serialize with collect_signal().
>>> */
>>> - if (unlikely(!list_empty(&q->list))) {
>>> - spinlock_t *lock = &current->sighand->siglock;
>>> - read_lock(&tasklist_lock);
>>> - spin_lock_irqsave(lock, flags);
>>> - if (!list_empty(&q->list))
>>> - list_del_init(&q->list);
>>> - spin_unlock_irqrestore(lock, flags);
>>> - read_unlock(&tasklist_lock);
>>> - }
>>> + spin_lock_irqsave(lock, flags);
>>> + if (!list_empty(&q->list))
>>> + list_del_init(&q->list);
>>> + spin_unlock_irqrestore(lock, flags);
>>> +
>>> q->flags &= ~SIGQUEUE_PREALLOC;
>>> __sigqueue_free(q);
>>> }
>>>
>>>
>>>
>>>
>> Applying previous patch???it seems likely that the __sigqueue_free() is
>> also called twice.
>>
>> collect_signal: sigqueue_free:
>>
>> list_del_init(&first->list);
>> spin_lock_irqsave(lock, flags);
>>
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>> if (!list_empty(&q->list))
>> list_del_init(&q->list);
>> spin_unlock_irqrestore(lock, flags);
>> q->flags &= ~SIGQUEUE_PREALLOC;
>>
>> __sigqueue_free(first); __sigqueue_free(q);
>>
>
> collect_signal() is always called under ->siglock which is also taken by
> sigqueue_free(), so this is not possible.
>
> Basically, this patch is the same one-liner I sent you before
>
> http://marc.info/?l=linux-kernel&m=118772206603453&w=2
>
> (Thanks for the additional testing and report, btw).
>
> P.S. It would be nice to know if this patch solves the problems reported
> by Jeremy, but his email is disabled.
>
> Oleg.
>
>
I know, using current->sighand->siglock to prevent one sigqueue
is free twice. I want to know whether it is possible that the two
function is called in different thread. If that, the spin_lock is useless.

yue.tao
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