Satyam Sharma writes: > > Doesn't "atomic WRT all processors" require volatility? > > No, it definitely doesn't. Why should it? > > "Atomic w.r.t. all processors" is just your normal, simple "atomicity" > for SMP systems (ensure that that object is modified / set / replaced > in main memory atomically) and has nothing to do with "volatile" > behaviour.
Atomic variables are "volatile" in the sense that they are liable to be changed at any time by mechanisms that are outside the knowledge of the C compiler, namely, other CPUs, or this CPU executing an interrupt routine. In the kernel we use atomic variables in precisely those situations where a variable is potentially accessed concurrently by multiple CPUs, and where each CPU needs to see updates done by other CPUs in a timely fashion. That is what they are for. Therefore the compiler must not cache values of atomic variables in registers; each atomic_read must result in a load and each atomic_set must result in a store. Anything else will just lead to subtle bugs. I have no strong opinion about whether or not the best way to achieve this is through the use of the "volatile" C keyword. Segher's idea of using asm instead seems like a good one to me. Paul. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
