Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 15 Aug 2007 09:27:22 -0700

On Wed, Aug 15, 2007 at 06:09:35PM +0200, Stefan Richter wrote:
> Herbert Xu wrote:
> > On Wed, Aug 15, 2007 at 08:05:38PM +0530, Satyam Sharma wrote:
> >>> I don't know if this here is affected:
> 
> [...something like]
> 	b = atomic_read(a);
> 	for (i = 0; i < 4; i++) {
> 		msleep_interruptible(63);
> 		c = atomic_read(a);
> 		if (c != b) {
> 			b = c;
> 			i = 0;
> 		}
> 	}
> 
> > Nope, we're calling schedule which is a rather heavy-weight
> > barrier.
> 
> How does the compiler know that msleep() has got barrier()s?

Because msleep_interruptible() is in a separate compilation unit,
the compiler has to assume that it might modify any arbitrary global.
In many cases, the compiler also has to assume that msleep_interruptible()
might call back into a function in the current compilation unit, thus
possibly modifying global static variables.

						Thanx, Paul