Re: [GIT PULL] RCU changes for v3.4

From: Linus Torvalds
Date: Sat Mar 24 2012 - 00:25:58 EST

On Fri, Mar 23, 2012 at 9:17 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> I must admit that __this_cpu_inc() would be nicer than __this_cpu_add(),
> though, will fix.  I need the leading "__" to avoid disabling preemption
> needlessly on non-x86 platforms.

Yeah, that's just bogus. Ok on that.

> The reason that the "__raw" forms are
> safe in this case is because the per-CPU variable is saved and restored
> at context-switch time.
> Or am I still missing something here?

It's not that the "__raw" forms are "safe". It's that they are SH*T.

Don't use them. They are crap. Why would you do

+ __raw_get_cpu_var(rcu_read_lock_nesting) =
+ current->rcu_read_lock_nesting_save;

which is just crazy and cannot use the actual sane "%fs:" segment
overrides, but instead has to do idiotic "ready the per-cpu offset
pointer and add it in".

We've got "__this_cpu_write()" which generates the correct code.

Rule of thumb: there is _never_ any good reason to use
__raw_get_cpu_var. It's a broken interface.

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