[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>

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index e193695..0bd8c76 100644 (file)
--- a/arch/powerpc/kernel/irq.c
+++ b/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();

diff --git a/include/asm-powerpc/hw_irq.h b/include/asm-powerpc/hw_irq.h
index c4a1ab6..fd3f2a2 100644 (file)
--- a/include/asm-powerpc/hw_irq.h
+++ b/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);