[ARM] 5440/1: Fix VFP state corruption due to preemption during VFP exceptions

We've observed that ARM VFP state can be corrupted during VFP exception
handling when PREEMPT is enabled.  The exact conditions are difficult
to reproduce but appear to occur during VFP exception handling when a
task causes a VFP exception which is handled via VFP_bounce and is then
preempted by yet another task which in turn causes yet another VFP
exception.  Since the VFP_bounce code is not preempt safe, VFP state then
becomes corrupt.  In order to prevent preemption from occuring while
handling a VFP exception, this patch disables preemption while handling
VFP exceptions.

Signed-off-by: George G. Davis <gdavis@mvista.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/vfp/entry.S

@@ -15,13 +15,16 @@
  *  r10 = thread_info structure
  *  lr  = failure return
  */
-#include <linux/linkage.h>
-#include <linux/init.h>
-#include <asm/asm-offsets.h>
-#include <asm/assembler.h>
+#include <asm/thread_info.h>
 #include <asm/vfpmacros.h>
+#include "../kernel/entry-header.S"
 
 ENTRY(do_vfp)
+#ifdef CONFIG_PREEMPT
+	ldr	r4, [r10, #TI_PREEMPT]	@ get preempt count
+	add	r11, r4, #1		@ increment it
+	str	r11, [r10, #TI_PREEMPT]
+#endif
 	enable_irq
  	ldr	r4, .LCvfp
 	ldr	r11, [r10, #TI_CPU]	@ CPU number
@@ -30,6 +33,12 @@ ENTRY(do_vfp)
 ENDPROC(do_vfp)
 
 ENTRY(vfp_null_entry)
+#ifdef CONFIG_PREEMPT
+	get_thread_info	r10
+	ldr	r4, [r10, #TI_PREEMPT]	@ get preempt count
+	sub	r11, r4, #1		@ decrement it
+	str	r11, [r10, #TI_PREEMPT]
+#endif
 	mov	pc, lr
 ENDPROC(vfp_null_entry)
 
@@ -41,6 +50,12 @@ ENDPROC(vfp_null_entry)
 
 	__INIT
 ENTRY(vfp_testing_entry)
+#ifdef CONFIG_PREEMPT
+	get_thread_info	r10
+	ldr	r4, [r10, #TI_PREEMPT]	@ get preempt count
+	sub	r11, r4, #1		@ decrement it
+	str	r11, [r10, #TI_PREEMPT]
+#endif
 	ldr	r0, VFP_arch_address
 	str	r5, [r0]		@ known non-zero value
 	mov	pc, r9			@ we have handled the fault

arch/arm/vfp/vfphw.S

@@ -137,6 +137,12 @@ check_for_exception:
 	VFPFMXR	FPEXC, r1		@ restore FPEXC last
 	sub	r2, r2, #4
 	str	r2, [sp, #S_PC]		@ retry the instruction
+#ifdef CONFIG_PREEMPT
+	get_thread_info	r10
+	ldr	r4, [r10, #TI_PREEMPT]	@ get preempt count
+	sub	r11, r4, #1		@ decrement it
+	str	r11, [r10, #TI_PREEMPT]
+#endif
 	mov	pc, r9			@ we think we have handled things
 
 
@@ -155,6 +161,12 @@ look_for_VFP_exceptions:
 	@ not recognised by VFP
 
 	DBGSTR	"not VFP"
+#ifdef CONFIG_PREEMPT
+	get_thread_info	r10
+	ldr	r4, [r10, #TI_PREEMPT]	@ get preempt count
+	sub	r11, r4, #1		@ decrement it
+	str	r11, [r10, #TI_PREEMPT]
+#endif
 	mov	pc, lr
 
 process_exception:

arch/arm/vfp/vfpmodule.c

@@ -266,7 +266,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
 		 * on VFP subarch 1.
 		 */
 		 vfp_raise_exceptions(VFP_EXCEPTION_ERROR, trigger, fpscr, regs);
-		 return;
+		goto exit;
 	}
 
 	/*
@@ -297,7 +297,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
 	 * the FPEXC.FP2V bit is valid only if FPEXC.EX is 1.
 	 */
 	if (fpexc ^ (FPEXC_EX | FPEXC_FP2V))
-		return;
+		goto exit;
 
 	/*
 	 * The barrier() here prevents fpinst2 being read
@@ -310,6 +310,8 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
 	exceptions = vfp_emulate_instruction(trigger, orig_fpscr, regs);
 	if (exceptions)
 		vfp_raise_exceptions(exceptions, trigger, orig_fpscr, regs);
+ exit:
+	preempt_enable();
 }
 
 static void vfp_enable(void *unused)