RE: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert()

["Kenn Humborg" <kenn@bluetree.ie>]

> On Thursday 16 August 2007 01:39, Satyam Sharma wrote:
> >
> >  static inline void wait_for_init_deassert(atomic_t *deassert)
> >  {
> > -	while (!atomic_read(deassert));
> > +	while (!atomic_read(deassert))
> > +		cpu_relax();
> >  	return;
> >  }
> 
> For less-than-briliant people like me, it's totally non-obvious that
> cpu_relax() is needed for correctness here, not just to make P4 happy.
> 
> IOW: "atomic_read" name quite unambiguously means "I will read
> this variable from main memory". Which is not true and creates
> potential for confusion and bugs.

To me, "atomic_read" means a read which is synchronized with other 
changes to the variable (using the atomic_XXX functions) in such 
a way that I will always only see the "before" or "after"
state of the variable - never an intermediate state while a 
modification is happening.  It doesn't imply that I have to 
see the "after" state immediately after another thread modifies
it.

Perhaps the Linux atomic_XXX functions work like that, or used
to work like that, but it's counter-intuitive to me that "atomic"
should imply a memory read.

Later,
Kenn