Re: [Boost-users] Thread cancellation

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2005-02-11 12:52:03

Peter Dimov wrote:
> Ben Hutchings wrote:
>> Paul wrote:
>>>> Synchronous cancellation may be simulated by setting a simple flag
>>>> polled by the cancellable thread.  (Appropriate memory barriers must
>>>> be placed after setting and before polling the flag.  In the absence
>>>> of portable memory barrier facilities, one may protect the flag
>>>> with a mutex, though that's annoyingly heavyweight.)  However this
>>>> does not
>>>
>>> just out of curiousity, why do you think you need a memory barrier
>>> on a flag?  you would have 1 writer and 1 reader...  either the
>>> reader reads false or true, and once the writer has set it to
>>> true... thats it, the reader will read true on the next pass.
>>
>> In practice, if you use "volatile" then the compiler will probably
>> generate code to read the flag from memory every time and the reader
>> will probably see the change, possibly after some small delay for it
>> to work its way through the write queue on the processor that the
>> writer is running on.  AFAIK no standard or specification guarantees
>> this.
> 
> If the cache of the reader CPU isn't notified about the change (which is 
> only guaranteed to happen on an acquire) the reader may never observe 
> the modified value.

I agree that there's no such guarantee, which is why I wouldn't advocate 
depending on this working.  However, without a cache coherency protocol, 
independent shared variables cannot share a cache line and 
synchronisation is absurdly slow, and I'm not aware of any 
multiprocessor systems supporting general multithreading that don't have 
such a protocol.  Write queues are of finite length and are eagerly 
flushed to cache to avoid stalling later memory-writing instructions 
where possible.

Ben.