Thread-safe way to add a key to a dict only if it isn't already there?
Marko Rauhamaa
marko at pacujo.net
Sun Jul 8 12:11:43 EDT 2018
MRAB <python at mrabarnett.plus.com>:
> On 2018-07-08 14:38, Steven D'Aprano wrote:
>> On Sun, 08 Jul 2018 14:11:58 +0300, Marko Rauhamaa wrote:
>>
> [snip]
>>> More importantly, this loop may never finish:
>>>
>>> # Initially
>>> quit = False
>>>
>>> # Thread 1
>>> global quit
>>> while not quit:
>>> time.sleep(1)
>>>
>>> # Thread 2
>>> global quit
>>> quit = True
>>
>> Assuming that thread 2 actually runs *at some point*, I don't see how
>> that can't terminate. Neither thread sets quit to False, so provided
>> thread 2 runs at all, it has to terminate.
>>
> [snip]
>
> The compiler could look at the code for thread 1 and see that 'quit' is
> never assigned to, meaning that it could be "optimised" to:
>
> global quit
> if not quit:
> while True:
> time.sleep(1)
>
> In C you'd declare 'quit' as 'volatile' to tell the compiler that it
> could change unexpectedly, so don't make that assumption.
C is an even tougher case. Even if the compiler kept on checking a
volatile value, the CPU might never propagate the cache content to the
other core. You'd need a memory barrier. In Java, "volatile" effectively
creates a memory barrier, but in C (and C++) it does not. In C you need
something like a mutex to see the effects of other threads running.
(BTW, I think that's a terrible thing for the C standards committee to
specify.)
Marko
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