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Subject[PATCH RFC tip/core/rcu 2/6] rcu: Inform the user about dynticks-idle mode on PROVE_RCU warning
Date
From: Frederic Weisbecker <fweisbec@gmail.com>

Inform the user if an RCU usage error is detected by lockdep while in
an extended quiescent state (in this case, dyntick-idle mode). This
is accomplished by adding a line to the RCU lockdep splat indicating
whether or not the splat occurred in dyntick-idle mode.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/lockdep.c | 20 ++++++++++++++++++++
1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 298c927..f2bff04 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3998,6 +3998,26 @@ void lockdep_rcu_dereference(const char *file, const int line)
file, line);
printk("\nother info that might help us debug this:\n\n");
printk("\nrcu_scheduler_active = %d, debug_locks = %d\n", rcu_scheduler_active, debug_locks);
+
+ /*
+ * If a CPU is in dyntick-idle mode (CONFIG_NO_HZ), then RCU
+ * considers that CPU to be in an "extended quiescent state",
+ * which means that RCU will be completely ignoring that CPU.
+ * Therefore, rcu_read_lock() and friends have absolutely no
+ * effect on a dyntick-idle CPU. In other words, even if a
+ * dyntick-idle CPU has called rcu_read_lock(), RCU might well
+ * delete data structures out from under it. RCU really has no
+ * choice here: if it were to consult the CPU, that would wake
+ * the CPU up, and the whole point of dyntick-idle mode is to
+ * allow CPUs to enter extremely deep sleep states.
+ *
+ * So complain bitterly if someone does call rcu_read_lock(),
+ * rcu_read_lock_bh() and so on from extended quiescent states
+ * such as dyntick-idle mode.
+ */
+ if (rcu_check_extended_qs())
+ printk("RCU used illegally from extended quiescent state!\n");
+
lockdep_print_held_locks(curr);
printk("\nstack backtrace:\n");
dump_stack();
--
1.7.3.2


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