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Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()
From: Steven Rostedt <rostedt@goodmis.org>
To: Christoph Lameter <cl@gentwo.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
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Date: Tue, 20 Sep 2011 14:54:01 -0400
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On Tue, 2011-09-20 at 11:10 -0500, Christoph Lameter wrote:
> On Tue, 20 Sep 2011, Steven Rostedt wrote:
> 
> > I really mean all other users of this_cpu_*(), including the cmpxchg and
> > friends, still need to have preemption disabled.
> 
> This is argument against the basic design of this_cpu_ops. They were
> designed to avoid having to disable preemption for single operations on
> per cpu data. I think this shows a basic misunderstanding of what you are
> dealing with.
> 

BTW, Can you explain to me where the this_cpu_*() ops were designed to
be used? The only places where "this_cpu_*()" is used in slub.c and
page_alloc.c have irqs disabled on their use. I thought this was for
slub and page_alloc?

Is this_cpu() made just for statistics? I see it used in the inode code
for that, and some accounting in the namespace.c code.

Note and there's places all over the kernel that uses this_cpu_read()
and thinks preemption should be disabled. Just look at
arch/x86/mm/tlb.c:

	/* Caller has disabled preemption */
	sender = this_cpu_read(tlb_vector_offset);

Why the comment?

My argument is that this_cpu_* is just confusing. Rename your use case
and keep this_cpu_*() as what you want __this_cpu_*() to be.

Thanks!

-- Steve