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[PATCH v3 15/15] x86/irq: Document how IRQ context tracking works and add an assertion

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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

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[PATCH v3 00/15] x86: Rewrite exit-to-userspace code Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200
  [PATCH v3 10/15] x86/asm/entry/64: Save all regs on interrupt entry Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200
  [PATCH v3 04/15] x86: Move C entry and exit code to arch/x86/entry/common.c Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200
  [PATCH v3 15/15] x86/irq: Document how IRQ context tracking works and add an assertion Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200
  [PATCH v3 13/15] x86/entry: Remove exception_enter from trap handlers Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200
  [PATCH v3 05/15] x86/traps: Assert that we're in CONTEXT_KERNEL in exception entries Andy Lutomirski <luto@kernel.org> - 2015-06-24 04:50 +0200

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