[ofw] [PATCH v2] complib/user: fix timer race conditions

[Fab Tillier]

Hefty, Sean wrote on Mon, 28 Jun 2010 at 15:34:51

>>> Moved the calculation of the timeout time to inside the critical
>>> section to improve its accuracy in case an attempt to acquire the
>>> critical section blocks.
>> 
>> How does this improve accuracy?  I suppose it depend on whether the
>> timeout time is relative to the client making the call, or the call
>> returning. Having the timeout calculated before the critsec improves
>> the former, calculating it under lock improves the latter.
> 
> I was worried about the time between setting timeout and using it:
> 
> +	timeout = cl_get_time_stamp() + (((uint64_t)time_ms) * 1000);
> +	if ( !p_timer->timeout_time || timeout < p_timer->timeout_time )
> 
> If timeout is set before the critical section, and the thread blocks,
> the if check is more likely to return success than setting it after.
> The result is that the timer may be adjusted, which would set the timer
> out _further_ that it actually is.

Ah, ok, that makes sense.

-Fab