Re: [Threads-devel] basic_timed_mutex::try_lock

From: Anthony Williams (anthony_at_[hidden])
Date: 2008-05-03 17:56:37

Quoting Dmitriy V'jukov <dvyukov_at_[hidden]>:

> boost version 1.35.0
>
> basic_timed_mutex::try_lock() implemented as:

[skip implementation]

> Why it always executes at least one
> BOOST_INTERLOCKED_COMPARE_EXCHANGE? In documentation I don't see any
> requirements that try_lock() have to synchronize memory in the case of
> failure. Why it's not implemented as:
>
>            bool try_lock()
>            {
>                long old_count=active_count;
>                while(!(old_count&lock_flag_value));
>                {
>                    long const current_count =
>                      BOOST_INTERLOCKED_COMPARE_EXCHANGE
>                          
> (&active_count,(old_count+1)|lock_flag_value,old_count);
>                    if(current_count==old_count)
>                    {
>                        return true;
>                    }
>                    old_count=current_count;
>                }
>                return false;
>            }

No particular reason. Thanks for the suggestion. Actually, I've been  
intending to change the implementation to use a BitTestAndSet  
instruction where it can, so try_lock becomes:

    return !bit_test_and_set(&active_count,lock_flag_bit);

But even then, it might be worth adding a simple non-interlocked read  
before-hand to check for the flag, and only do the BTS if it's not set.

Thanks,

Anthony
-- 
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