On Tue, 27 Nov 2007 23:35:36 +0100, Arnd Bergmann wrote:

On Monday 26 November 2007, Jan Kratochvil wrote:

quoted

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!

This sounds like a bug recently reported by Uli Weigand. BenH
said he'd take a look, but it probably fell under the table.
The problem found by Uli is that on certain processors (Cell/B.E.
in his case), the DABRX register needs to be set in order for
the DABR to take effect.

Please be aware DABR works fine if the same code runs just 1 (always) or
2 (sometimes) threads.  It starts failing with too many threads running:

$ ./dabr-lost
TID 32725: DABR 0x1001279f NIP 0xfecf41c
TID 32726: DABR 0x1001279f NIP 0xfecf41c
TID 32725: hitting the variable
variable found = -1, caught TID = 32725
TID 32726: hitting the variable
variable found = -1, caught TID = 32726
The kernel bug did not get reproduced - increase THREADS.

As I did not find any code in that kernel touching DABRX its value should not
be dependent on the number of threads running.


Regards,
Lace