x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/e8bdc4ed0193fb2fd130f3d6b7b8023e2ec1ab62.1435952415.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 88b36648..6233de0 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -216,8 +216,23 @@
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)) {