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Subject[ 64/85] powerpc: Restore correct DSCR in context switch
Date
From: Greg KH <gregkh@linuxfoundation.org>

3.4-stable review patch. If anyone has any objections, please let me know.

------------------

From: Anton Blanchard <anton@samba.org>

commit 714332858bfd40dcf8f741498336d93875c23aa7 upstream.

During a context switch we always restore the per thread DSCR value.
If we aren't doing explicit DSCR management
(ie thread.dscr_inherit == 0) and the default DSCR changed while
the process has been sleeping we end up with the wrong value.

Check thread.dscr_inherit and select the default DSCR or per thread
DSCR as required.

This was found with the following test case, when running with
more threads than CPUs (ie forcing context switching):

http://ozlabs.org/~anton/junkcode/dscr_default_test.c

With the four patches applied I can run a combination of all
test cases successfully at the same time:

http://ozlabs.org/~anton/junkcode/dscr_default_test.c
http://ozlabs.org/~anton/junkcode/dscr_explicit_test.c
http://ozlabs.org/~anton/junkcode/dscr_inherit_test.c

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/powerpc/kernel/asm-offsets.c | 1 +
arch/powerpc/kernel/entry_64.S | 23 +++++++++++++++++------
2 files changed, 18 insertions(+), 6 deletions(-)

--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -76,6 +76,7 @@ int main(void)
DEFINE(SIGSEGV, SIGSEGV);
DEFINE(NMI_MASK, NMI_MASK);
DEFINE(THREAD_DSCR, offsetof(struct thread_struct, dscr));
+ DEFINE(THREAD_DSCR_INHERIT, offsetof(struct thread_struct, dscr_inherit));
#else
DEFINE(THREAD_INFO, offsetof(struct task_struct, stack));
#endif /* CONFIG_PPC64 */
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -373,6 +373,12 @@ _GLOBAL(ret_from_fork)
li r3,0
b syscall_exit

+ .section ".toc","aw"
+DSCR_DEFAULT:
+ .tc dscr_default[TC],dscr_default
+
+ .section ".text"
+
/*
* This routine switches between two different tasks. The process
* state of one is saved on its kernel stack. Then the state
@@ -512,9 +518,6 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEG
mr r1,r8 /* start using new stack pointer */
std r7,PACAKSAVE(r13)

- ld r6,_CCR(r1)
- mtcrf 0xFF,r6
-
#ifdef CONFIG_ALTIVEC
BEGIN_FTR_SECTION
ld r0,THREAD_VRSAVE(r4)
@@ -523,14 +526,22 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_PPC64
BEGIN_FTR_SECTION
+ lwz r6,THREAD_DSCR_INHERIT(r4)
+ ld r7,DSCR_DEFAULT@toc(2)
ld r0,THREAD_DSCR(r4)
- cmpd r0,r25
- beq 1f
+ cmpwi r6,0
+ bne 1f
+ ld r0,0(r7)
+1: cmpd r0,r25
+ beq 2f
mtspr SPRN_DSCR,r0
-1:
+2:
END_FTR_SECTION_IFSET(CPU_FTR_DSCR)
#endif

+ ld r6,_CCR(r1)
+ mtcrf 0xFF,r6
+
/* r3-r13 are destroyed -- Cort */
REST_8GPRS(14, r1)
REST_10GPRS(22, r1)



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