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

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Date:	Fri, 23 Mar 2012 21:25:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU changes for v3.4

On Fri, Mar 23, 2012 at 9:17 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> 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.

                       Linus
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