[PATCH perf/core] uprobes: remove too strict lockdep_assert() condition in hprobe_expire()

[Andrii Nakryiko]

hprobe_expire() is used to atomically switch pending uretprobe instance
(struct return_instance) from being SRCU protected to be refcounted.
This can be done from background timer thread, or synchronously within
current thread when task is forked.

In the former case, return_instance has to be protected through RCU read
lock, and that's what hprobe_expire() used to check with
lockdep_assert(rcu_read_lock_held()).

But in the latter case (hprobe_expire() called from dup_utask()) there
is no RCU lock being held, and it's both unnecessary and incovenient.
Inconvenient due to the intervening memory allocations inside
dup_return_instance()'s loop. Unnecessary because dup_utask() is called
synchronously in current thread, and no uretprobe can run at that point,
so return_instance can't be freed either.

So drop rcu_read_lock_held() condition, and expand corresponding comment
to explain necessary lifetime guarantees. lockdep_assert()-detected
issue is a false positive.

Fixes: dd1a7567784e ("uprobes: SRCU-protect uretprobe lifetime (with timeout)")
Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 kernel/events/uprobes.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index e783da1d1762..4d2140cab7ec 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -762,10 +762,14 @@ static struct uprobe *hprobe_expire(struct hprobe *hprobe, bool get)
 	enum hprobe_state hstate;
 
 	/*
-	 * return_instance's hprobe is protected by RCU.
-	 * Underlying uprobe is itself protected from reuse by SRCU.
+	 * Caller should guarantee that return_instance is not going to be
+	 * freed from under us. This can be achieved either through holding
+	 * rcu_read_lock() or by owning return_instance in the first place.
+	 *
+	 * Underlying uprobe is itself protected from reuse by SRCU, so ensure
+	 * SRCU lock is held properly.
 	 */
-	lockdep_assert(rcu_read_lock_held() && srcu_read_lock_held(&uretprobes_srcu));
+	lockdep_assert(srcu_read_lock_held(&uretprobes_srcu));
 
 	hstate = READ_ONCE(hprobe->state);
 	switch (hstate) {
-- 
2.43.5