[PATCH] rcu: Debug nasty rcu user mode API recursion

[Frederic Weisbecker]

Add some debug code to chase down the origin of the fault.

Not-Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
---
 arch/x86/mm/fault.c      |    1 +
 include/linux/rcupdate.h |    1 +
 kernel/rcutree.c         |   32 ++++++++++++++++++++++++++++++++
 kernel/rcutree.h         |    1 +
 4 files changed, 35 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a530b23..a5f0eb5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1232,6 +1232,7 @@ good_area:
 dotraplinkage void __kprobes
 do_page_fault(struct pt_regs *regs, unsigned long error_code)
 {
+	rcu_check_user_recursion(regs);
 	exception_enter(regs);
 	__do_page_fault(regs, error_code);
 	exception_exit(regs);
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 7c968e4..14ba908 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -199,6 +199,7 @@ extern void rcu_user_enter_after_irq(void);
 extern void rcu_user_exit_after_irq(void);
 extern void rcu_user_hooks_switch(struct task_struct *prev,
 				  struct task_struct *next);
+extern void rcu_check_user_recursion(struct pt_regs *regs);
 #else
 static inline void rcu_user_enter(void) { }
 static inline void rcu_user_exit(void) { }
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 4fb2376..63b84f5 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -405,6 +405,20 @@ void rcu_idle_enter(void)
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
 #ifdef CONFIG_RCU_USER_QS
+void rcu_check_user_recursion(struct pt_regs *regs)
+{
+	unsigned long flags;
+	static int printed;
+
+	local_irq_save(flags);
+	if (__this_cpu_read(rcu_dynticks.recursion) && !printed) {
+		printed = 1;
+		printk("Found recursion\n");
+		show_regs(regs);
+	}
+	local_irq_restore(flags);
+}
+
 /**
  * rcu_user_enter - inform RCU that we are resuming userspace.
  *
@@ -433,10 +447,20 @@ void rcu_user_enter(void)
 
 	local_irq_save(flags);
 	rdtp = &__get_cpu_var(rcu_dynticks);
+	if (WARN_ON_ONCE(rdtp->recursion)) {
+		local_irq_restore(flags);
+		return;
+	}
+
+	rdtp->recursion = true;
+	barrier();
+
 	if (!rdtp->ignore_user_qs && !rdtp->in_user) {
 		rdtp->in_user = true;
 		rcu_eqs_enter(true);
 	}
+	rdtp->recursion = false;
+
 	local_irq_restore(flags);
 }
 
@@ -590,10 +614,18 @@ void rcu_user_exit(void)
 
 	local_irq_save(flags);
 	rdtp = &__get_cpu_var(rcu_dynticks);
+	if (WARN_ON_ONCE(rdtp->recursion)) {
+		local_irq_restore(flags);
+		return;
+	}
+
+	rdtp->recursion = true;
+	barrier();
 	if (rdtp->in_user) {
 		rdtp->in_user = false;
 		rcu_eqs_exit(true);
 	}
+	rdtp->recursion = false;
 	local_irq_restore(flags);
 }
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 5faf05d..1bde9d5 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -103,6 +103,7 @@ struct rcu_dynticks {
 	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
 #ifdef CONFIG_RCU_USER_QS
+	bool recursion;
 	bool ignore_user_qs;	    /* Treat userspace as extended QS or not */
 	bool in_user;		    /* Is the CPU in userland from RCU POV? */
 #endif
-- 
1.7.9.5

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