[PATCH] perf: Use raw_smp_processor_id insted of smp_processor_id

On Wed, Sep 12, 2012 at 11:44:29AM -0700, Stephen Boyd wrote:

On 09/12/12 05:05, Russell King - ARM Linux wrote:

quoted

NAK.  Using a different function which doesn't have the warning isn't a
subsitute for fixing the problem properly.  What you're doing is papering
over the bug, making the warning go away without properly understanding
the problem.

The warning is there because something is being done wrong.  What that is
is exactly what is being said in the warning message.  You're getting a
CPU number in a context where preemptions can occur - and therefore the
CPU that you're running on can change.

Think about this sequence:

	- cpu = smp_processor_id(); /* returns 0 */
	- you are preempted
	- you resume on CPU 1
	- trace_clock_disable(clk->name, 0, 0);

If trace_clock_disable() uses the CPU number to access per-CPU data
without locking, that's going to cause corruption.

Please, if you're going to fix a warning, analyse it properly first and
don't just search for a function which appears to give you the same
functionality but without the warning message.

Is anyone actually using the CPU field in this tracepoint?

No idea, but for the point I'm raising, that's rather irrelevant... we
need to nip these things in the bud before they become more common
through allowing the misunderstanding to propagate.

I don't see
any usecase for it except for the case where you have a percpu clock,

Which actually makes it all the more important to get the right CPU
number, and to have atomicity between reading the CPU number and reading
the percpu clock... without that the results may well be meaningless.

If the use of a CPU number can be eliminated, then it should be.  If not,
the proper fix for this is to use interfaces which prevent pre-emption
occuring during the critical region - so get_cpu() and put_cpu().