PPC upstream kernel ignored DABR bug

[Jan Kratochvil]

Hi,

this testcase:
	http://people.redhat.com/jkratoch/dabr-lost.c

reproduces a PPC DABR kernel bug.  The variable `variable' should not get
modified as the thread modifying it should be caught by its DABR:

$ ./dabr-lost
TID 30914: DABR 0x10012a77 NIP 0x80f6ebb318
TID 30915: DABR 0x10012a77 NIP 0x80f6ebb318
TID 30916: DABR 0x10012a77 NIP 0x80f6ebb318
TID 30914: hitting the variable
TID 30915: hitting the variable
TID 30916: hitting the variable
variable found = 30916, caught TID = 30914
TID 30916: DABR 0x10012a77
Variable got modified by a thread which has DABR still set!

At the `variable found =' line the parent ptracer found the TID thread 30916
wrote the value into the variable - despite it had DABR alrady set before.

As the behavior is dependent on the current weather I expect the scheduling
matters there.

It is important the target thread is in the `nanosleep' syscall.  If you define
WORKAROUND_SET_DABR_IN_SYSCALL in the testcase it busyloops in the userland and
the bug gets no longer reproduced.

I got it reproduced on a utrace-patched kernel on dual-CPU Power5 and Roland
McGrath reported it reproduced on the vanilla upstream kernel on a Mac G5.



Regards,
Jan Kratochvil