Re: [Boost-users] [atomic] ThreadSanitizer reports a data race when using the code from "reference counting" example

Subject: Re: [Boost-users] [atomic] ThreadSanitizer reports a data race when using the code from "reference counting" example
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-07-01 08:15:07

On 1 Jul 2014 at 13:34, Adam Romanek wrote:

> See above for my private conversation with Andrey Semashev on this topic 
> (he does not follow the boost-users mailing list).
> 
> Andrey says that the "reference counting example" from Boost.Atomic docs 
> is broken. What do you think about this?

I wouldn't be surprised. Few have access to non-Intel hardware for 
testing, and all Intel CPUs always acquire loads and always release 
stores. That makes use of atomics with anything but 
memory_order_seq_cst superfluous on Intel at the CPU level.

Additionally, modern CPUs do very well with memory_order_seq_cst 
across the board, even an ARM Cortex A15. If I ever find myself 
tempted to not use memory_order_seq_cst before benchmarking I usually 
slap myself hard. The only real problem with its use is that 
compilers basically turn off optimisation around them, so you'd 
rather not have them in tight loops if possible.

The ONLY thing which might penalise this approach in the future is 
memory transactions - I can see memory_order_seq_cst would have to 
abort more frequently than other orderings. But until such hardware 
capability is here, I really wouldn't worry. If you see problem code 
not using memory_order_seq_cst, change it to that and bypass the 
hassle.

Niall

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