[PATCH] powerpc/64: Don't trace code that runs with the soft irq mask unreconciled

[Nicholas Piggin]

"Reconciling" in terms of interrupt handling, is to bring the soft irq
mask state in to synch with the hardware, after an interrupt causes
MSR[EE] to be cleared (while the soft mask may be enabled, and hard
irqs not marked disabled).

General kernel code should not be called while unreconciled, because
local_irq_disable, etc. manipulations can cause surprising irq traces,
and it's fragile because the soft irq code does not really expect to
be called in this situation.

When exiting from an interrupt, MSR[EE] is cleared to prevent races,
but soft irq state is enabled for the returned-to context, so this is
now an unreconciled state. restore_math is called in this state, and
that can be ftraced, and the ftrace subsystem disables local irqs.

Mark restore_math and its callees as notrace. Restore a sanity check
in the soft irq code that had to be disabled for this case, by commit
4da1f79227ad4 ("powerpc/64: Disable irq restore warning for now").

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/kernel/fpu.S     |  1 +
 arch/powerpc/kernel/irq.c     | 13 +++----------
 arch/powerpc/kernel/process.c | 18 +++++++++++++++---
 arch/powerpc/kernel/vector.S  |  1 +
 4 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
index 529dcc21c3f9..cecd57e1d046 100644
--- a/arch/powerpc/kernel/fpu.S
+++ b/arch/powerpc/kernel/fpu.S
@@ -63,6 +63,7 @@ _GLOBAL(load_fp_state)
 	REST_32FPVSRS(0, R4, R3)
 	blr
 EXPORT_SYMBOL(load_fp_state)
+_ASM_NOKPROBE_SYMBOL(load_fp_state); /* used by restore_math */
 
 /*
  * Store FP state into memory, including FPSCR
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index 8a936723c791..083934ecabb2 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -261,16 +261,9 @@ notrace void arch_local_irq_restore(unsigned long mask)
 	 */
 	irq_happened = get_irq_happened();
 	if (!irq_happened) {
-		/*
-		 * FIXME. Here we'd like to be able to do:
-		 *
-		 * #ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
-		 *   WARN_ON(!(mfmsr() & MSR_EE));
-		 * #endif
-		 *
-		 * But currently it hits in a few paths, we should fix those and
-		 * enable the warning.
-		 */
+#ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
+		WARN_ON(!(mfmsr() & MSR_EE));
+#endif
 		return;
 	}
 
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index f7b2e3b3db28..c4279e1a4a38 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -133,7 +133,8 @@ static int __init enable_strict_msr_control(char *str)
 }
 early_param("ppc_strict_facility_enable", enable_strict_msr_control);
 
-unsigned long msr_check_and_set(unsigned long bits)
+/* notrace because it's called by restore_math */
+unsigned long notrace msr_check_and_set(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -152,7 +153,8 @@ unsigned long msr_check_and_set(unsigned long bits)
 }
 EXPORT_SYMBOL_GPL(msr_check_and_set);
 
-void __msr_check_and_clear(unsigned long bits)
+/* notrace because it's called by restore_math */
+void notrace __msr_check_and_clear(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -525,7 +527,17 @@ void giveup_all(struct task_struct *tsk)
 }
 EXPORT_SYMBOL(giveup_all);
 
-void restore_math(struct pt_regs *regs)
+/*
+ * The exception exit path calls restore_math() with interrupts hard disabled
+ * but the soft irq state not "reconciled". ftrace code that calls
+ * local_irq_save/restore causes warnings.
+ *
+ * Rather than complicate the exit path, just don't trace restore_math. This
+ * could be done by having ftrace entry code check for this un-reconciled
+ * condition where MSR[EE]=0 and PACA_IRQ_HARD_DIS is not set, and
+ * temporarily fix it up for the duration of the ftrace call.
+ */
+void notrace restore_math(struct pt_regs *regs)
 {
 	unsigned long msr;
 
diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
index 21165da0052d..8eb867dbad5f 100644
--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S
@@ -21,6 +21,7 @@ _GLOBAL(load_vr_state)
 	REST_32VRS(0,r4,r3)
 	blr
 EXPORT_SYMBOL(load_vr_state)
+_ASM_NOKPROBE_SYMBOL(load_vr_state); /* used by restore_math */
 
 /*
  * Store VMX state into memory, including VSCR.
-- 
2.20.1