linux-kernel - Re: Q: lockdep_assert_held_read() after downgrade

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Message-ID: <20170131112526.GH6536@twins.programming.kicks-ass.net>
Date:   Tue, 31 Jan 2017 12:25:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jens Axboe <axboe@...com>
Cc:     "J. R. Okajima" <hooanon05g@...il.com>,
        linux-kernel@...r.kernel.org, darrick.wong@...cle.com,
        david@...morbit.com, longman@...hat.com, dave@...olabs.net
Subject: Re: Q: lockdep_assert_held_read() after downgrade_write()

On Tue, Jan 31, 2017 at 11:36:20AM +0100, Peter Zijlstra wrote:
> On Mon, Jan 30, 2017 at 02:30:45PM -0700, Jens Axboe wrote:

> > I don't think you understand how it works. downgrade_write() turns a write
> > lock into read held. To make that last sequence valid, you'd need:
> 
> Correct, and I'm surprised that didn't explode in different ways.
> 
> > 
> > 	down_write(&rw);
> > 	downgrade_write(&rw);
> > 	lockdep_assert_held_read(&rw)
> > 	up_read(&rw);
> > 
> > or just not drop up_write() from the last section.
> 
> Right, but also, there seems to be a missing lockdep annotation to make
> that work. That is, downgrade_write() doesn't have a lockdep annotation,
> so it (lockdep) will still think its a write lock.
> 
> 
> Let me try and fix both issues.

Something like so I suppose,... completely untested.

There could be a good reason for the current lockdep behaviour, but I
cannot remember.

---
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 45ba475d4be3..dfa9e40f83d5 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -123,10 +123,9 @@ EXPORT_SYMBOL(up_write);
  */
 void downgrade_write(struct rw_semaphore *sem)
 {
-	/*
-	 * lockdep: a downgraded write will live on as a write
-	 * dependency.
-	 */
+	rwsem_release(&sem->dep_map, 1, _RET_IP_);
+	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
+
 	rwsem_set_reader_owned(sem);
 	__downgrade_write(sem);
 }
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index a699f4048ba1..3bd584c81b0b 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -40,8 +40,10 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 	 * do a write to the rwsem cacheline when it is really necessary
 	 * to minimize cacheline contention.
 	 */
-	if (sem->owner != RWSEM_READER_OWNED)
+	if (sem->owner != RWSEM_READER_OWNED) {
+		WARN_ON_ONCE(sem->owner != current);
 		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+	}
 }
 
 static inline bool rwsem_owner_is_writer(struct task_struct *owner)