[ltt-dev] [PATCH] urcu-qsbr: avoid useless futex wakeups and burning CPU for long grace periods

Paolo Bonzini pbonzini at redhat.com
Wed Aug 17 03:39:28 EDT 2011

On 08/16/2011 05:29 AM, Mathieu Desnoyers wrote:
>> >  +		:: (in_registry[0] == 0&&  in_registry[1] == 0) ->
>> >  +progress:
>> >  +#ifndef INJ_LATE_DEC
>> >  +			gp_futex = 0;
>> >  +#endif
>> >  +			gp = !gp;
> Shouldn't you flip the gp for both steps of synchronize_rcu ? Here, it
> looks like you only flip gp after the reader passes through both of the
> checks, which defeats the purpose the 2-phase grace period.

Other comments are more or less trivial (i.e. you are right :)), so I'll 
just answer a few.

Here, I flip gp after both readers have passed through a grace period. 
Note that the waiter process doesn't model synchronize_rcu, it models 

This is because if you have only one writer, and no threads going 
online/offline, there is no difference between an infinite sequence of 
synchronize_rcu and an infinite sequence of update_counter_and_wait: the 
latter is just two copies of the former, back to back.

> for consistency, you seem to use "in_registry[1]" and "in_registry[1] ==
> 1" for pretty much the same thing. Is there a reason why they differ ?

No, I should always use == 1.


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