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.
