[PATCH] perf: Use raw_smp_processor_id insted of smp_processor_id

Russell King - ARM Linux linux at arm.linux.org.uk
Wed Sep 12 08:05:11 EDT 2012

On Wed, Sep 12, 2012 at 02:50:10PM +0300, Roger Quadros wrote:
> gets rid of below messages with CONFIG_DEBUG_PREEMPT enabled
> [   28.832916] debug_smp_processor_id: 18 callbacks suppressed
> [   28.832946] BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/1763
> [   28.841491] caller is pwrdm_set_next_pwrst+0x54/0x120
> Tested with perf on OMAP4 Panda.

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.

More information about the linux-arm-kernel mailing list