[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, 6 insertions, 0 deletions

diff --git a/arch/ia64/kernel/ptrace.c b/arch/ia64/kernel/ptrace.c
index 08c8a5e..575a8f6 100644
--- a/arch/ia64/kernel/ptrace.c
+++ b/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();
 }
 
 /*