[POWERPC] Make soft_enabled irqs preempt safe

Rewrite local_get_flags and local_irq_disable to use r13 explicitly,
to avoid the risk that gcc will split get_paca()->soft_enabled into a
sequence unsafe against preemption.  Similar care in local_irq_restore.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -97,22 +97,69 @@ EXPORT_SYMBOL(irq_desc);
 
 int distribute_irqs = 1;
 
+static inline unsigned long get_hard_enabled(void)
+{
+	unsigned long enabled;
+
+	__asm__ __volatile__("lbz %0,%1(13)"
+	: "=r" (enabled) : "i" (offsetof(struct paca_struct, hard_enabled)));
+
+	return enabled;
+}
+
+static inline void set_soft_enabled(unsigned long enable)
+{
+	__asm__ __volatile__("stb %0,%1(13)"
+	: : "r" (enable), "i" (offsetof(struct paca_struct, soft_enabled)));
+}
+
 void local_irq_restore(unsigned long en)
 {
-	get_paca()->soft_enabled = en;
+	/*
+	 * get_paca()->soft_enabled = en;
+	 * Is it ever valid to use local_irq_restore(0) when soft_enabled is 1?
+	 * That was allowed before, and in such a case we do need to take care
+	 * that gcc will set soft_enabled directly via r13, not choose to use
+	 * an intermediate register, lest we're preempted to a different cpu.
+	 */
+	set_soft_enabled(en);
 	if (!en)
 		return;
 
 	if (firmware_has_feature(FW_FEATURE_ISERIES)) {
-		if (get_paca()->lppaca_ptr->int_dword.any_int)
+		/*
+		 * Do we need to disable preemption here?  Not really: in the
+		 * unlikely event that we're preempted to a different cpu in
+		 * between getting r13, loading its lppaca_ptr, and loading
+		 * its any_int, we might call iseries_handle_interrupts without
+		 * an interrupt pending on the new cpu, but that's no disaster,
+		 * is it?  And the business of preempting us off the old cpu
+		 * would itself involve a local_irq_restore which handles the
+		 * interrupt to that cpu.
+		 *
+		 * But use "local_paca->lppaca_ptr" instead of "get_lppaca()"
+		 * to avoid any preemption checking added into get_paca().
+		 */
+		if (local_paca->lppaca_ptr->int_dword.any_int)
 			iseries_handle_interrupts();
 		return;
 	}
 
-	if (get_paca()->hard_enabled)
+	/*
+	 * if (get_paca()->hard_enabled) return;
+	 * But again we need to take care that gcc gets hard_enabled directly
+	 * via r13, not choose to use an intermediate register, lest we're
+	 * preempted to a different cpu in between the two instructions.
+	 */
+	if (get_hard_enabled())
 		return;
-	/* need to hard-enable interrupts here */
-	get_paca()->hard_enabled = en;
+
+	/*
+	 * Need to hard-enable interrupts here.  Since currently disabled,
+	 * no need to take further asm precautions against preemption; but
+	 * use local_paca instead of get_paca() to avoid preemption checking.
+	 */
+	local_paca->hard_enabled = en;
 	if ((int)mfspr(SPRN_DEC) < 0)
 		mtspr(SPRN_DEC, 1);
 	hard_irq_enable();

include/asm-powerpc/hw_irq.h

@@ -18,15 +18,25 @@ extern void timer_interrupt(struct pt_regs *);
 
 static inline unsigned long local_get_flags(void)
 {
-	return get_paca()->soft_enabled;
+	unsigned long flags;
+
+	__asm__ __volatile__("lbz %0,%1(13)"
+	: "=r" (flags)
+	: "i" (offsetof(struct paca_struct, soft_enabled)));
+
+	return flags;
 }
 
 static inline unsigned long local_irq_disable(void)
 {
-	unsigned long flag = get_paca()->soft_enabled;
-	get_paca()->soft_enabled = 0;
-	barrier();
-	return flag;
+	unsigned long flags, zero;
+
+	__asm__ __volatile__("li %1,0; lbz %0,%2(13); stb %1,%2(13)"
+	: "=r" (flags), "=&r" (zero)
+	: "i" (offsetof(struct paca_struct, soft_enabled))
+	: "memory");
+
+	return flags;
 }
 
 extern void local_irq_restore(unsigned long);