[PATCH v4 17/17] x86/irq: Document how IRQ context tracking works and add an assertion

Andy Lutomirski
2015-06-29T19:36:23Z

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/kernel/irq.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 88b366487b0e..6233de046c08 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -216,8 +216,23 @@ __visible unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
unsigned vector = ~regs->orig_ax;
unsigned irq;

+ /*
+ * NB: Unlike exception entries, IRQ entries do not reliably
+ * handle context tracking in the low-level entry code. This is
+ * because syscall entries execute briefly with IRQs on before
+ * updating context tracking state, so we can take an IRQ from
+ * kernel mode with CONTEXT_USER. The low-level entry code only
+ * updates the context if we came from user mode, so we won't
+ * switch to CONTEXT_KERNEL. We'll fix that once the syscall
+ * code is cleaned up enough that we can cleanly defer enabling
+ * IRQs.
+ */
+
entering_irq();

+ /* entering_irq() tells RCU that we're not quiescent. Check it. */
+ rcu_lockdep_assert(rcu_is_watching(), "IRQ failed to wake up RCU");
+
irq = __this_cpu_read(vector_irq[vector]);

if (!handle_irq(irq, regs)) {
--
2.4.3