rcu: Annotate SRCU's update-side lockdep dependencies

Although all flavors of RCU readers are annotated correctly with
lockdep as recursive read locks, they do not set the lock_acquire
'check' parameter.  This means that RCU read locks are not added to
the lockdep dependency graph, which in turn means that lockdep cannot
detect RCU-based deadlocks.  This is not a problem for RCU flavors having
atomic read-side critical sections because context-based annotations can
catch these deadlocks, see for example the RCU_LOCKDEP_WARN() statement
in synchronize_rcu().  But context-based annotations are not helpful
for sleepable RCU, especially given that it is perfectly legal to do
synchronize_srcu(&srcu1) within an srcu_read_lock(&srcu2).

However, we can detect SRCU-based by: (1) Making srcu_read_lock() a
'check'ed recursive read lock and (2) Making synchronize_srcu() a empty
write lock critical section.  Even better, with the newly introduced
lock_sync(), we can avoid false positives about irq-unsafe/safe.
This commit therefore makes it so.

Note that NMI-safe SRCU read side critical sections are currently not
annotated, but might be annotated in the future.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ boqun: Add comments for annotation per Waiman's suggestion ]
[ boqun: Fix comment warning reported by Stephen Rothwell ]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index b12fb0cec44d..336af24e0fe3 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -197,6 +197,8 @@ void synchronize_srcu(struct srcu_struct *ssp)
 {
 	struct rcu_synchronize rs;
 
+	srcu_lock_sync(&ssp->dep_map);
+
 	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
 			lock_is_held(&rcu_bh_lock_map) ||
 			lock_is_held(&rcu_lock_map) ||