[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>

3 files changed, 35 insertions, 6 deletions

diff --git a/arch/arm/vfp/entry.S b/arch/arm/vfp/entry.S
index ba592a9e6fb3..a2bed62aec21 100644
--- a/arch/arm/vfp/entry.S
+++ b/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
diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index a5a4e57763c3..83c4e384b16d 100644
--- a/arch/arm/vfp/vfphw.S
+++ b/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:
diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index 75457b30d813..01599c4ef726 100644
--- a/arch/arm/vfp/vfpmodule.c
+++ b/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)