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Subject[RFC PATCH 2/2] Documentation/locking/lockdep: Add section about available annotations
Date
Add section about annotations that can be used to perform additional runtime
checking of locking correctness: assert that certain locks are held and
prevent accidental unlocking.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
---
Documentation/locking/lockdep-design.txt | 47 ++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index e341c2f34e68..74347a24efc7 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -169,6 +169,53 @@ Note: When changing code to use the _nested() primitives, be careful and
check really thoroughly that the hierarchy is correctly mapped; otherwise
you can get false positives or false negatives.

+Annotations
+-----------
+
+Two constructs can be used to annotate and check where and if certain locks
+must be held: lockdep_assert_held*(&lock) and lockdep_*pin_lock(&lock).
+
+As the name suggests, lockdep_assert_held* family of macros assert that a
+particular lock is held at a certain time (and generate a WARN otherwise).
+This annotation is largely used all over the kernel, e.g. kernel/sched/
+core.c
+
+ void update_rq_clock(struct rq *rq)
+ {
+ s64 delta;
+
+ lockdep_assert_held(&rq->lock);
+ [...]
+ }
+
+where holding rq->lock is required to safely update a rq's clock.
+
+The other family of macros is lockdep_*pin_lock, which is admittedly only
+used for rq->lock ATM. Despite their limited adoption these annotations
+generate a WARN if the lock of interest is "accidentally" unlocked. This turns
+out to be especially helpful to debug code with callbacks, where an upper
+layer assumes a lock remains taken, but a lower layer thinks it can maybe drop
+and reacquire the lock ("unwittingly" introducing races). lockdep_pin_lock
+returns a 'struct pin_cookie' that is then used by lockdep_unpin_lock to check
+that nobody tampered with the lock, e.g. kernel/sched/sched.h
+
+ static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf)
+ {
+ rf->cookie = lockdep_pin_lock(&rq->lock);
+ [...]
+ }
+
+ static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)
+ {
+ [...]
+ lockdep_unpin_lock(&rq->lock, rf->cookie);
+ }
+
+While comments about locking requirements might provide useful information,
+the runtime checks performed by annotations are invaluable when debugging
+locking problems and they carry the same level of details when inspecting
+code. Always prefer annotations when in doubt!
+
Proof of 100% correctness:
--------------------------

--
2.14.3
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