[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>

arch/ia64/kernel/ptrace.c

@@ -635,11 +635,17 @@ ia64_flush_fph (struct task_struct *task)
 {
 	struct ia64_psr *psr = ia64_psr(ia64_task_regs(task));
 
+	/*
+	 * Prevent migrating this task while
+	 * we're fiddling with the FPU state
+	 */
+	preempt_disable();
 	if (ia64_is_local_fpu_owner(task) && psr->mfh) {
 		psr->mfh = 0;
 		task->thread.flags |= IA64_THREAD_FPH_VALID;
 		ia64_save_fpu(&task->thread.fph[0]);
 	}
+	preempt_enable();
 }
 
 /*

arch/ia64/kernel/traps.c

@@ -220,13 +220,21 @@ disabled_fph_fault (struct pt_regs *regs)
 
 	/* first, grant user-level access to fph partition: */
 	psr->dfh = 0;
+
+	/*
+	 * Make sure that no other task gets in on this processor
+	 * while we're claiming the FPU
+	 */
+	preempt_disable();
 #ifndef CONFIG_SMP
 	{
 		struct task_struct *fpu_owner
 			= (struct task_struct *)ia64_get_kr(IA64_KR_FPU_OWNER);
 
-		if (ia64_is_local_fpu_owner(current))
+		if (ia64_is_local_fpu_owner(current)) {
+			preempt_enable_no_resched();
 			return;
+		}
 
 		if (fpu_owner)
 			ia64_flush_fph(fpu_owner);
@@ -244,6 +252,7 @@ disabled_fph_fault (struct pt_regs *regs)
 		 */
 		psr->mfh = 1;
 	}
+	preempt_enable_no_resched();
 }
 
 static inline int

include/asm-ia64/processor.h

@@ -403,15 +403,21 @@ 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);						\
 	(__ia64_islfo_task->thread.last_fph_cpu == smp_processor_id()				\
 	 && __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();			\