Re: [PATCH v3] ipc/mqueue: Avoid relying on a stack reference past its expiry

[Davidlohr Bueso <dbueso@suse.de>]

On 2021-05-08 12:23, Manfred Spraul wrote:
> Hi Varad,
> 
> On 5/7/21 3:38 PM, Varad Gautam wrote:
>> @@ -1005,11 +1022,9 @@ static inline void __pipelined_op(struct 
>> wake_q_head *wake_q,
>>   				  struct ext_wait_queue *this)
>>   {
>>   	list_del(&this->list);
>> -	get_task_struct(this->task);
>> -
>> +	wake_q_add(wake_q, this->task);
>>   	/* see MQ_BARRIER for purpose/pairing */
>>   	smp_store_release(&this->state, STATE_READY);
>> -	wake_q_add_safe(wake_q, this->task);
>>   }
>>     /* pipelined_send() - send a message directly to the task waiting 
>> in
> 
> First, I was too fast: I had assumed that wake_q_add() before
> smp_store_release() would be a potential lost wakeup.

Yeah you need wake_up_q() to actually wake anything up.

> 
> As __pipelined_op() is called within spin_lock(&info->lock), and as
> wq_sleep() will reread this->state after acquiring
> spin_lock(&info->lock), I do not see a bug anymore.

Right, and when I proposed this version of the fix I was mostly focusing 
on STATE_READY
being set as the last operation, but the fact of the matter is we had 
moved to the
wake_q_add_safe() version for two reasons:

(1) Ensuring the ->state = STATE_READY is done after the reference count 
and avoid
racing with exit. In mqueue's original use of wake_q we were relying on 
the call's
implied barrier from wake_q_add() in order to avoid reordering of 
setting the state.
But this turned out to be insufficient hence the explicit 
smp_store_release().

(2) In order to prevent a potential lost wakeup when the blocked task is 
already queued
for wakeup by another task (the failed cmpxchg case in wake_q_add), and 
therefore we need
to set the return condition (->state = STATE_READY) before adding the 
task to the wake_q.

But I'm not seeing how race (2) can happen in mqueue. The race was 
always theoretical to
begin with, with the exception of rwsems[1] in which actually the wakee 
task could end up in
the waker's wake_q without actually blocking.

So all in all I now agree that we should keep the order of how we 
currently have things,
just to be on the safer side, if nothing else.

[1] 
https://lore.kernel.org/lkml/1543495830-2644-1-git-send-email-xieyongji@baidu.com

Thanks,
Davidlohr