Commit 05062d96 authored by Peter Chubb's avatar Peter Chubb Committed by Linus Torvalds
Browse files

[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: default avatarPeter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent f829fd23

arch/ia64/kernel/ptrace.c

+6 −0
Original line number Diff line number Diff line
@@ -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

+10 −1
Original line number Diff line number Diff line
@@ -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

+8 −2
Original line number Diff line number Diff line
@@ -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();			\

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