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authorJuri Lelli <juri.lelli@redhat.com>2018-02-13 19:55:19 +0100
committerIngo Molnar <mingo@kernel.org>2018-02-14 12:01:22 +0100
commita1ea544fe0911492b9f8d101bcbf46cc8c47fbc5 (patch)
tree6ec7c592250f11c9f0118982ce141d1aeae30fd0 /Documentation/locking
parente5684bbfc3f03480d6ba2150f133630fb510d3eb (diff)
Documentation/locking/lockdep: Add section about available annotations
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> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-doc@vger.kernel.org Link: http://lkml.kernel.org/r/20180213185519.18186-3-juri.lelli@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'Documentation/locking')
-rw-r--r--Documentation/locking/lockdep-design.txt47
1 files changed, 47 insertions, 0 deletions
diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index e341c2f34e68..49f58a07ee7b 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:
--------------------------