[PATCH v5 1/5] rv/reactors: use LD_WAIT_SPIN as the reactor lockdep wait type

[wen . yang]

From: Wen Yang <wen.yang@xxxxxxxxx>

rv_react() overrides the lockdep wait type to LD_WAIT_FREE to enforce
that reactor callbacks take no locks. But callbacks run in the context
of the triggering tracepoint, which can be preemptible task context on
any kernel. A timer interrupt firing during the callback makes the
interrupt-exit path schedule and take rq->__lock (LD_WAIT_SPIN) while
the LD_WAIT_FREE override is still held, producing a spurious
"Invalid wait context" warning:

    [ BUG: Invalid wait context ]
    context-{5:5}
    1 lock held by kunit_try_catch/209:
     #0: (rv_react_map-wait-type-override){+.+.}-{1:1}
    kunit_try_catch/209 is trying to lock:
    ffff8a743ed3e8a0 (&rq->__lock){-...}-{2:2}

Use LD_WAIT_SPIN instead of LD_WAIT_FREE, which causes false-positive
warnings in preemptible contexts due to scheduler preemption taking
rq->__lock. Add documentation to runtime-verification.rst.

Fixes: 69d8895cb9a9 ("rv: Add explicit lockdep context for reactors")
Reviewed-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
Signed-off-by: Wen Yang <wen.yang@xxxxxxxxx>
Cc: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
---
 kernel/trace/rv/rv_reactors.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/rv/rv_reactors.c b/kernel/trace/rv/rv_reactors.c
index 2f5fc8d18dea..ff7d478227c3 100644
--- a/kernel/trace/rv/rv_reactors.c
+++ b/kernel/trace/rv/rv_reactors.c
@@ -465,7 +465,15 @@ int init_rv_reactors(struct dentry *root_dir)
 
 void rv_react(struct rv_monitor *monitor, const char *msg, ...)
 {
-	static DEFINE_WAIT_OVERRIDE_MAP(rv_react_map, LD_WAIT_FREE);
+	/*
+	 * Reactors must not explicitly take locks, so they should be
+	 * LD_WAIT_FREE.  However, reactor callbacks can run with preemption
+	 * enabled, meaning the preempting code (e.g. the scheduler taking
+	 * rq->__lock at LD_WAIT_SPIN) may violate that constraint.  Use
+	 * LD_WAIT_SPIN to avoid false-positive lockdep reports.
+	 * But you should still NOT be using locks in reactors.
+	 */
+	static DEFINE_WAIT_OVERRIDE_MAP(rv_react_map, LD_WAIT_SPIN);
 	va_list args;
 
 	if (!rv_reacting_on() || !monitor->react)
-- 
2.25.1