Re: [PATCH] drivers: cpuidle: Disable preemption before get_lppaca function call in pseries_idle_probe function

From: Victor Aoqui
Date: Tue Aug 01 2017 - 08:27:09 EST


Em 2017-07-20 18:21, Benjamin Herrenschmidt escreveu:
On Thu, 2017-07-20 at 14:57 -0300, Victor Aoqui wrote:
When CONFIG_PREEMPT=y, the following warning shows up:

BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
caller is pseries_processor_idle_init+0x58/0x21c

This warning shows up because preemption cannot occur when using
get_paca(), otherwise the paca_struct it points to may be the wrong one
just after.

For this reason, preemption needs to be disabled before
lppaca_shared_proc(get_lppaca()).

Also chekc the generated assembly. We had all sort of interesting
issues where gcc would copy the paca pointer or the lppaca pointer
to a GPR *outside* of the preempt disabled section...

In that specific case it's not a big deal but overall, I am not
comfortable with PREEMPT on powerpc until we do something a bit
more drastic...

I would like to remove all such direct accesses to paca, instead have a
"new" get_paca() written in asm that does the preempt disable then
returns the PACA in a GPR (not directly use r13, hide that from gcc),
and which is paired with a put_paca().

The few places where we want to directly access r13 should be hand
written in asm too to hide r13 from gcc, for accessing the irq_happened
in the fast path of local_irq_enable/disable/... we should do the same
with lock tokens.

Ben.

Hi Benjamin,

Sorry for the delay. I was a little bit busy last days.
I took note of your comments and I will work on those changes.
I will let you know soon when it's done.

Thanks

--
Victor Aoqui