Close a race relating to setting the PCPU pointer (r13). Register r13

points to the TLS in user space and points to the PCPU structure in
the kernel. The race is the result of having the exception handler on
the one hand and the RPC system call entry on the other. The EPC
syscall path is non-atomic in that interrupts are enabled while the
two stacks are switched. The register stack is switched last as that
is the stack used to determine whether we're going back to user space
by the exception handler. If we go back to user space, we restore r13,
otherwise we leave r13 alone. The EPC syscall path however set r13 to
the PCPU structure *before* switching the register stack, which means
that there was a window in which the exception handler would restore
r13 when it was already pointing to the PCPU structure. This is fatal
when the exception happened on CPU x, but left from the exception on
anotehr CPU. In that case r13 would point to the PCPU of the CPU the
thread was running on. This immediately results in getting the wrong
value for curthread.
The fix is to make sure we assign r13 *after* we set ar.bspstore to
point to the kernel register stack for the thread.

1 files changed, 5 insertions, 5 deletions

diff --git a/sys/ia64/ia64/syscall.S b/sys/ia64/ia64/syscall.S
index ef559e8..84a03da 100644
--- a/sys/ia64/ia64/syscall.S
+++ b/sys/ia64/ia64/syscall.S
@@ -228,7 +228,7 @@ ENTRY_NOPROFILE(epc_syscall, 8)
 {	.mmi
 	mov		r16=ar.rsc
 	mov		ar.rsc=0
-	mov		r17=r13
+	nop		0
 	;;
 }
 {	.mmi
@@ -239,13 +239,13 @@ ENTRY_NOPROFILE(epc_syscall, 8)
 	;;
 }
 {	.mmi
-	mov		r13=ar.k4
+	mov		r21=ar.unat
 	add		r30=-SIZEOF_TRAPFRAME,r14
 	mov		r20=sp
 	;;
 }
 {	.mii
-	mov		r21=ar.unat
+	mov		r17=r13
 	dep		r30=0,r30,0,10
 	;;
 	add		sp=-16,r30
@@ -258,10 +258,10 @@ ENTRY_NOPROFILE(epc_syscall, 8)
 	add		r31=8,r30
 	;;
 }
-{	.mii
+{	.mmi
+	mov		r13=ar.k4
 	mov		r22=ar.fpsr
 	sub		r29=r14,r30
-	nop		0
 }
 {	.mmi
 	mov		r23=ar.bsp