linux-kernel - Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read as read

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Date:   Fri, 3 Sep 2021 12:45:57 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read
 as read

On 2021-09-02 00:43:45 [+0800], Boqun Feng wrote:
> If a reader is recursive, then a pending writer doesn't block the
> recursive reader, otherwise, a pending write blocks the reader. IOW, a
> pending writer blocks non-recursive readers but not recursive readers.

Puh. So I would describe it as writer fair but maybe I'm not fluent in
locking. But you don't mean recursive reader as in 

   T1			T2
   read_lock(a);
			write_lock(a);
   read_lock(a);

which results in a deadlock (but T1 recursively acquired the `a' lock). 

However, PREEMPT_RT's locking implementation always blocks further
reader from entering locked section once a writer is pending so that
would then ask for something like this:

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5572,16 +5572,19 @@ static bool lockdep_nmi(void)
 }
 
 /*
- * read_lock() is recursive if:
- * 1. We force lockdep think this way in selftests or
- * 2. The implementation is not queued read/write lock or
- * 3. The locker is at an in_interrupt() context.
+ * read_lock() is recursive if the implementation allows readers to enter the
+ * locked section even if a writer is pending. This is case if:
+ * - We force lockdep think this way in selftests
+ * - The implementation is queued read/write lock and the locker is in
+ *   in_interrupt() context.
+ * - Non queued read/write implementation allow it unconditionally.
+ * - PREEMPT_RT's implementation never allows it.
  */
 bool read_lock_is_recursive(void)
 {
 	return force_read_lock_recursive ||
-	       !IS_ENABLED(CONFIG_QUEUED_RWLOCKS) ||
-	       in_interrupt();
+	       (IS_ENABLED(CONFIG_QUEUED_RWLOCKS) && in_interrupt()) ||
+	       !IS_ENABLED(CONFIG_PREEMPT_RT);
 }
 EXPORT_SYMBOL_GPL(read_lock_is_recursive);
 
Sebastian