RFC Introduce checks for preemptable code for this_cpu_read/write()

I just found out that the this_cpu_*() functions do not perform the test to see if the usage is in atomic or not. Thus, the blind conversion of the per_cpu(*, smp_processor_id()) and the get_cpu_var() code to this_cpu_*() introduce the regression to detect the hard to find case where a per cpu variable is used in preempt code that migrates and causes bugs.

I'm adding back a simple call to smp_processor_id() to this_cpu_read() and this_cpu_write() to see what errors I find. I found 4 places that required touch up. 3 were areas that the race is known and we don't care, but the memcg looks very buggy.

Anyway, if I had known that the this_cpu_*() code removed this smp_processor_id() check, I would have NAK'd all the changes to the tracing code.

I only modified this_cpu_read and *_write() I may decide other this_cpu_* code should be updated too. Especially the ones that return a value.

-- Steve

Steven Rostedt (5):
x86: Remove const_udelay() caring about which cpu var it uses
mm: Switch mod_state() to __this_cpu_read()
memcg: Disable preemption in memcg_check_events()
printk: Have wake_up_klogd() use __this_cpu_write()
percpu: Add preempt checks back into this_cpu_read/write()

----

 arch/x86/lib/delay.c   |    2 +-
 include/linux/percpu.h |   17 +++++++++++++++--
 kernel/printk.c        |    2 +-
 mm/memcontrol.c        |    2 ++
 mm/vmstat.c            |    4 ++--
 5 files changed, 21 insertions(+), 6 deletions(-)