[PATCH] ia64: fix floating-point preemption problem

There've been reports of problems with CONFIG_PREEMPT=y and the high
floating point partition.  This is caused by the possibility of preemption
and rescheduling on a different processor while saving or restioirng the
high partition.

The only places where the FPU state is touched are in ptrace, in
switch_to(), and where handling a floating-point exception.  In switch_to()
preemption is off.  So it's only in trap.c and ptrace.c that we need to
prevent preemption.

Here is a patch that adds commentary to make the conditions clear, and adds
appropriate preempt_{en,dis}able() calls to make it so.  In trap.c I use
preempt_enable_no_resched(), as we're about to return to user space where
the preemption flag will be checked anyway.

Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/asm-ia64/processor.h b/include/asm-ia64/processor.h
index 9e1ba8b7fb68..91bbd1f22461 100644
--- a/include/asm-ia64/processor.h
+++ b/include/asm-ia64/processor.h
@@ -403,7 +403,10 @@ extern void ia64_setreg_unknown_kr (void);
  * task_struct at this point.
  */
 
-/* Return TRUE if task T owns the fph partition of the CPU we're running on. */
+/*
+ * Return TRUE if task T owns the fph partition of the CPU we're running on.
+ * Must be called from code that has preemption disabled.
+ */
 #define ia64_is_local_fpu_owner(t)                                                              \
 ({                                                                                              \
         struct task_struct *__ia64_islfo_task = (t);                                            \
@@ -411,7 +414,10 @@ extern void ia64_setreg_unknown_kr (void);
          && __ia64_islfo_task == (struct task_struct *) ia64_get_kr(IA64_KR_FPU_OWNER));        \
 })
 
-/* Mark task T as owning the fph partition of the CPU we're running on. */
+/*
+ * Mark task T as owning the fph partition of the CPU we're running on.
+ * Must be called from code that has preemption disabled.
+ */
 #define ia64_set_local_fpu_owner(t) do {                                                \
         struct task_struct *__ia64_slfo_task = (t);                                     \
         __ia64_slfo_task->thread.last_fph_cpu = smp_processor_id();                     \