The following commit has been merged into the locking/core branch of tip:
Commit-ID: 5f2962401c6e195222f320d12b3a55377b2d4653
Gitweb:
https://git.kernel.org/tip/5f2962401c6e195222f320d12b3a55377b2d4653
Author: Boqun Feng <boqun.f...@gmail.com>
AuthorDate: Thu, 10 Dec 2020 11:15:00 +01:00
Committer: Peter Zijlstra <pet...@infradead.org>
CommitterDate: Thu, 14 Jan 2021 11:20:17 +01:00
locking/lockdep: Exclude local_lock_t from IRQ inversions
The purpose of local_lock_t is to abstract: preempt_disable() /
local_bh_disable() / local_irq_disable(). These are the traditional
means of gaining access to per-cpu data, but are fundamentally
non-preemptible.
local_lock_t provides a per-cpu lock, that on !PREEMPT_RT reduces to
no-ops, just like regular spinlocks do on UP.
This gives rise to:
CPU0 CPU1
local_lock(B) spin_lock_irq(A)
<IRQ>
spin_lock(A) local_lock(B)
Where lockdep then figures things will lock up; which would be true if
B were any other kind of lock. However this is a false positive, no
such deadlock actually exists.
For !RT the above local_lock(B) is preempt_disable(), and there's
obviously no deadlock; alternatively, CPU0's B != CPU1's B.
For RT the argument is that since local_lock() nests inside
spin_lock(), it cannot be used in hardirq context, and therefore CPU0
cannot in fact happen. Even though B is a real lock, it is a
preemptible lock and any threaded-irq would simply schedule out and
let the preempted task (which holds B) continue such that the task on
CPU1 can make progress, after which the threaded-irq resumes and can
finish.
This means that we can never form an IRQ inversion on a local_lock
dependency, so terminate the graph walk when looking for IRQ
inversions when we encounter one.
One consequence is that (for LOCKDEP_SMALL) when we look for redundant
dependencies, A -> B is not redundant in the presence of A -> L -> B.
Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
[peterz: Changelog]
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
kernel/locking/lockdep.c | 57 ++++++++++++++++++++++++++++++++++++---
1 file changed, 53 insertions(+), 4 deletions(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index f2ae8a6..ad9afd8 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2200,6 +2200,44 @@ static inline bool usage_match(struct lock_list *entry,
void *mask)
return !!((entry->class->usage_mask & LOCKF_IRQ) & *(unsigned
long *)mask);
}
+static inline bool usage_skip(struct lock_list *entry, void *mask)
+{
+ /*
+ * Skip local_lock() for irq inversion detection.
+ *
+ * For !RT, local_lock() is not a real lock, so it won't carry any
+ * dependency.
+ *
+ * For RT, an irq inversion happens when we have lock A and B, and on
+ * some CPU we can have:
+ *
+ * lock(A);
+ * <interrupted>
+ * lock(B);
+ *
+ * where lock(B) cannot sleep, and we have a dependency B -> ... -> A.
+ *
+ * Now we prove local_lock() cannot exist in that dependency. First we
+ * have the observation for any lock chain L1 -> ... -> Ln, for any
+ * 1 <= i <= n, Li.inner_wait_type <= L1.inner_wait_type, otherwise
+ * wait context check will complain. And since B is not a sleep lock,
+ * therefore B.inner_wait_type >= 2, and since the inner_wait_type of
+ * local_lock() is 3, which is greater than 2, therefore there is no
+ * way the local_lock() exists in the dependency B -> ... -> A.
+ *
+ * As a result, we will skip local_lock(), when we search for irq
+ * inversion bugs.
+ */
+ if (entry->class->lock_type == LD_LOCK_PERCPU) {
+ if (DEBUG_LOCKS_WARN_ON(entry->class->wait_type_inner <
LD_WAIT_CONFIG))
+ return false;
+
+ return true;
+ }
+
+ return false;
+}
+
/*
* Find a node in the forwards-direction dependency sub-graph starting
* at @root->class that matches @bit.
@@ -2215,7 +2253,7 @@ find_usage_forwards(struct lock_list *root, unsigned long
usage_mask,
debug_atomic_inc(nr_find_usage_forwards_checks);
- result = __bfs_forwards(root, &usage_mask, usage_match, NULL,
target_entry);
+ result = __bfs_forwards(root, &usage_mask, usage_match, usage_skip,
target_entry);
return result;
}
@@ -2232,7 +2270,7 @@ find_usage_backwards(struct lock_list *root, unsigned
long usage_mask,
debug_atomic_inc(nr_find_usage_backwards_checks);
- result = __bfs_backwards(root, &usage_mask, usage_match, NULL,
target_entry);
+ result = __bfs_backwards(root, &usage_mask, usage_match, usage_skip,
target_entry);
return result;
}
@@ -2597,7 +2635,7 @@ static int check_irq_usage(struct task_struct *curr,
struct held_lock *prev,
*/
bfs_init_rootb(&this, prev);
- ret = __bfs_backwards(&this, &usage_mask, usage_accumulate, NULL, NULL);
+ ret = __bfs_backwards(&this, &usage_mask, usage_accumulate, usage_skip,
NULL);
if (bfs_error(ret)) {
print_bfs_bug(ret);
return 0;
@@ -2664,6 +2702,12 @@ static inline int check_irq_usage(struct task_struct
*curr,
{
return 1;
}
+
+static inline bool usage_skip(struct lock_list *entry, void *mask)
+{
+ return false;
+}
+
#endif /* CONFIG_TRACE_IRQFLAGS */
#ifdef CONFIG_LOCKDEP_SMALL
@@ -2697,7 +2741,12 @@ check_redundant(struct held_lock *src, struct held_lock
*target)
debug_atomic_inc(nr_redundant_checks);
- ret = check_path(target, &src_entry, hlock_equal, NULL, &target_entry);
+ /*
+ * Note: we skip local_lock() for redundant check, because as the
+ * comment in usage_skip(), A -> local_lock() -> B and A -> B are not
+ * the same.
+ */
+ ret = check_path(target, &src_entry, hlock_equal, usage_skip,
&target_entry);
if (ret == BFS_RMATCH)
debug_atomic_inc(nr_redundant);
