linux-kernel - [PATCH v2] rcu: mollify sparse with RCU guard

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Date: Mon, 25 Mar 2024 18:39:08 +0100
From: Johannes Berg <johannes@...solutions.net>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH v2] rcu: mollify sparse with RCU guard

From: Johannes Berg <johannes.berg@...el.com>

When using "guard(rcu)();" sparse will complain, because even
though it now understands the cleanup attribute, it doesn't
evaluate the calls from it at function exit, and thus doesn't
count the context correctly.

Given that there's a conditional in the resulting code:

  static inline void class_rcu_destructor(class_rcu_t *_T)
  {
      if (_T->lock) {
          rcu_read_unlock();
      }
  }

it seems that even trying to teach sparse to evalulate the
cleanup attribute function it'd still be difficult to really
make it understand the full context here.

Suppress the sparse warning by just releasing the context in
the acquisition part of the function, after all we know it's
safe with the guard, that's the whole point of it.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
v2: add a comment after discussion with Boqun

---
 include/linux/rcupdate.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..6a3c52b3c180 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1059,6 +1059,18 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
 extern int rcu_expedited;
 extern int rcu_normal;
 
-DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
+DEFINE_LOCK_GUARD_0(rcu,
+		    do {
+			rcu_read_lock();
+			/*
+			 * sparse doesn't call the cleanup function,
+			 * so just release immediately and don't track
+			 * the context. We don't need to anyway, since
+			 * the whole point of the guard is to not need
+			 * the explicit unlock.
+			 */
+			__release(RCU);
+		    } while(0),
+		    rcu_read_unlock())
 
 #endif /* __LINUX_RCUPDATE_H */
-- 
2.44.0