ARM: hw_breakpoint: disable preemption during debug exception handling

On ARM, debug exceptions occur in the form of data or prefetch aborts.
One difference is that debug exceptions require access to per-cpu banked
registers and data structures which are not saved in the low-level exception
code. For kernels built with CONFIG_PREEMPT, there is an unlikely scenario
that the debug handler ends up running on a different CPU from the one
that originally signalled the event, resulting in random data being read
from the wrong registers.

This patch adds a debug_entry macro to the low-level exception handling
code which checks whether the taken exception is a debug exception. If
it is, the preempt count for the faulting process is incremented. After
the debug handler has finished, the count is decremented.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm/kernel/entry-header.S

@@ -165,6 +165,25 @@
 	.endm
 #endif	/* !CONFIG_THUMB2_KERNEL */
 
+	@
+	@ Debug exceptions are taken as prefetch or data aborts.
+	@ We must disable preemption during the handler so that
+	@ we can access the debug registers safely.
+	@
+	.macro	debug_entry, fsr
+#if defined(CONFIG_HAVE_HW_BREAKPOINT) && defined(CONFIG_PREEMPT)
+	ldr	r4, =0x40f		@ mask out fsr.fs
+	and	r5, r4, \fsr
+	cmp	r5, #2			@ debug exception
+	bne	1f
+	get_thread_info r10
+	ldr	r6, [r10, #TI_PREEMPT]	@ get preempt count
+	add	r11, r6, #1		@ increment it
+	str	r11, [r10, #TI_PREEMPT]
+1:
+#endif
+	.endm
+
 /*
  * These are the registers used in the syscall handler, and allow us to
  * have in theory up to 7 arguments to a function - r0 to r6.