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Subject: Re: [PATCH 7/7] rust: file: add abstraction for `poll_table`
From:   Alice Ryhl <aliceryhl@google.com>
To:     boqun.feng@gmail.com
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Boqun Feng <boqun.feng@gmail.com> writes:
>> That said, `synchronize_rcu` is rather expensive and is not needed in
>> all cases: If we have never registered a `poll_table` with the
>> `wait_list`, then we don't need to call `synchronize_rcu`. (And this is
>> a common case in Binder - not all processes use Binder with epoll.) The
>> current implementation does not account for this, but we could change it
>> to store a boolean next to the `wait_list` to keep track of whether a
>> `poll_table` has ever been registered. It is up to discussion whether
>> this is desireable.
>> 
>> It is not clear to me whether we can implement the above without storing
>> an extra boolean. We could check whether the `wait_list` is empty, but
>> it is not clear that this is sufficient. Perhaps someone knows the
>> answer? If a `poll_table` has previously been registered with a
> 
> That won't be sufficient, considering this:
> 
>     CPU 0                           CPU 1
>                                     ep_remove_wait_queue():
>                                       whead = smp_load_acquire(&pwq->whead); // whead is not NULL
>     PollCondVar::drop():
>       self.inner.notify():
>         <for each wait entry in the list>
> 	  ep_poll_callback():
> 	    <remove wait entry>
>             smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
>       <lock the waitqueue>
>       waitqueue_active() // return false, since the queue is emtpy
>       <unlock>
>     ...
>     <free the waitqueue>
> 				       if (whead) {
> 				         remove_wait_queue(whead, &pwq->wait); // Use-after-free BOOM!
> 				       }
>       
> 
> Note that moving the `wait_list` empty checking before
> `self.inner.notify()` won't change the result, since there might be a
> `notify` called by users before `PollCondVar::drop()`, hence the same
> result.
> 
> Regards,
> Boqun

Thank you for confirming my suspicion.

Alice