powerpc: Restore correct DSCR in context switch

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>
Cc: <stable@kernel.org> # 3.0+
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 17 insertions, 6 deletions

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 4b01a25e29ef..b40e0b4815b3 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -370,6 +370,12 @@ _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
@@ -509,9 +515,6 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
         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)
@@ -520,14 +523,22 @@ 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)