Re: [boost] Lightweight Mutex Code

From: Aaron W. LaFramboise (aaronrabiddog51_at_[hidden])
Date: 2004-07-02 17:41:47

Tyson Whitehead wrote:
> I believe there is a mistake in the 'details/lwm_gcc.hpp' and 
> 'details/lwm_linux.hpp' (and possibly the other lightweight_mutex 
> implementation as well -- I haven't looked at them).

I am not sure what the status of these are.  Are other components
supposed to be using these?  In addition to what you mentioned, they're
broken in other ways:

> In both of these files we have a counter (m_.a_) that is initialized to 1.  
> The lock routine is implemented as follows:
> 
> while( !__exchange_and_add(&m_.a_, -1) ){
>   __atomic_add(&m_.a_, 1);
>   sched_yield();

If a low priority thread is preempted by a high priority thread
immediately before __atomic_add, and the high priority thread is
attempting to grab the spinlock, deadlock is acheived, despite the
sched_yield().

I think that spinlocks should not be used in a way other than as an
optimization for the non-contended case in an algorithm with fallback to
a real synchronization primative.  I think it is a bug to do otherwise
on most non-realtime systems.

Aaron W. LaFramboise