We can skip adding a dependency 'AX -> B', in case that we ensure 'AX ->
the previous of B in hlocks' to be created, where AX is a crosslock and
B is a typical lock. Remember that two adjacent locks in hlocks generate
a dependency like 'prev -> next', that is, 'the previous of B in hlocks
-> B' in this case.
For example:
in hlocks[]
------------
^ A (gen_id: 4) --+
| | previous gen_id
| B (gen_id: 3) <-+
| C (gen_id: 3)
| D (gen_id: 2)
oldest | E (gen_id: 1)
in xhlocks[]
------------
^ A (gen_id: 4, prev_gen_id: 3(B's gen id))
| B (gen_id: 3, prev_gen_id: 3(C's gen id))
| C (gen_id: 3, prev_gen_id: 2(D's gen id))
| D (gen_id: 2, prev_gen_id: 1(E's gen id))
oldest | E (gen_id: 1, prev_gen_id: NA)
On commit for a crosslock AX(gen_id = 3), it's engough to add 'AX -> C',
but adding 'AX -> B' and 'AX -> A' is unnecessary since 'AX -> C', 'C ->
B' and 'B -> A' cover them, which are guaranteed to be generated.
This patch intoduces a variable, prev_gen_id, to avoid adding this kind
of redundant dependencies. In other words, the previous in hlocks will
anyway handle it if the previous's gen_id >= the crosslock's gen_id.
Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
include/linux/lockdep.h | 11 +++++++++++
kernel/locking/lockdep.c | 32 ++++++++++++++++++++++++++++++--
2 files changed, 41 insertions(+), 2 deletions(-)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index f7c730a..e5c5cc4 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -284,6 +284,17 @@ struct held_lock {
*/
struct hist_lock {
/*
+ * We can skip adding a dependency 'a target crosslock -> this
+ * lock', in case that we ensure 'the target crosslock -> the
+ * previous lock in held_locks' to be created. Remember that
+ * 'the previous lock in held_locks -> this lock' is guaranteed
+ * to be created, and 'A -> B' and 'B -> C' cover 'A -> C'.
+ *
+ * Keep the previous's gen_id to make the decision.
+ */
+ unsigned int prev_gen_id;
+
+ /*
* Id for each entry in the ring buffer. This is used to
* decide whether the ring buffer was overwritten or not.
*
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 09f5eec..a14d2ca 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4778,7 +4778,7 @@ static inline int xhlock_valid(struct hist_lock *xhlock)
*
* Irq disable is only required.
*/
-static void add_xhlock(struct held_lock *hlock)
+static void add_xhlock(struct held_lock *hlock, unsigned int prev_gen_id)
{
unsigned int idx = ++current->xhlock_idx;
struct hist_lock *xhlock = &xhlock(idx);
@@ -4793,6 +4793,11 @@ static void add_xhlock(struct held_lock *hlock)
/* Initialize hist_lock's members */
xhlock->hlock = *hlock;
+ /*
+ * prev_gen_id is used to skip adding redundant dependencies,
+ * which can be covered by the previous lock in held_locks.
+ */
+ xhlock->prev_gen_id = prev_gen_id;
xhlock->hist_id = current->hist_id++;
xhlock->trace.nr_entries = 0;
@@ -4813,6 +4818,11 @@ static inline int same_context_xhlock(struct hist_lock *xhlock)
*/
static void check_add_xhlock(struct held_lock *hlock)
{
+ struct held_lock *prev;
+ struct held_lock *start;
+ unsigned int gen_id;
+ unsigned int gen_id_invalid;
+
/*
* Record a hist_lock, only in case that acquisitions ahead
* could depend on the held_lock. For example, if the held_lock
@@ -4822,7 +4832,22 @@ static void check_add_xhlock(struct held_lock *hlock)
if (!current->xhlocks || !depend_before(hlock))
return;
- add_xhlock(hlock);
+ gen_id = (unsigned int)atomic_read(&cross_gen_id);
+ /*
+ * gen_id_invalid should be old enough to be invalid.
+ * Current gen_id - (UINIT_MAX / 4) would be a good
+ * value to meet it.
+ */
+ gen_id_invalid = gen_id - (UINT_MAX / 4);
+ start = current->held_locks;
+
+ for (prev = hlock - 1; prev >= start &&
+ !depend_before(prev); prev--);
+
+ if (prev < start)
+ add_xhlock(hlock, gen_id_invalid);
+ else if (prev->gen_id != gen_id)
+ add_xhlock(hlock, prev->gen_id);
}
/*
@@ -4979,6 +5004,9 @@ static void commit_xhlocks(struct cross_lock *xlock)
prev_hist_id = xhlock->hist_id;
+ if (!before(xhlock->prev_gen_id, xlock->hlock.gen_id))
+ continue;
+
/*
* commit_xhlock() returns 0 with graph_lock already
* released if fail.
--
1.9.1
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