From a1ea544fe0911492b9f8d101bcbf46cc8c47fbc5 Mon Sep 17 00:00:00 2001 From: Juri Lelli Date: Tue, 13 Feb 2018 19:55:19 +0100 Subject: [PATCH] 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 Acked-by: Peter Zijlstra (Intel) Cc: Jonathan Corbet Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-doc@vger.kernel.org Link: http://lkml.kernel.org/r/20180213185519.18186-3-juri.lelli@redhat.com Signed-off-by: Ingo Molnar --- 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..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: --------------------------