linux-kernel - Re: [PATCH v3 1/4] locking: Add rwsem_assert_held() and rwsem_assert_held

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Date:   Tue, 14 Nov 2023 21:26:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J . Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
        Mateusz Guzik <mjguzik@...il.com>
Subject: Re: [PATCH v3 1/4] locking: Add rwsem_assert_held() and
 rwsem_assert_held_write()

On Fri, Nov 10, 2023 at 05:21:22PM -0500, Waiman Long wrote:
> On 11/10/23 15:41, Matthew Wilcox (Oracle) wrote:
> >   static inline int rwsem_is_locked(struct rw_semaphore *sem)
> >   {
> > -	return atomic_long_read(&sem->count) != 0;
> > +	return atomic_long_read(&sem->count) != RWSEM_UNLOCKED_VALUE;
> >   }
> > -#define RWSEM_UNLOCKED_VALUE		0L
> > -#define __RWSEM_COUNT_INIT(name)	.count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
> > +static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
> > +{
> > +	WARN_ON(atomic_long_read(&sem->count) == RWSEM_UNLOCKED_VALUE);
> > +}
> That is not correct. You mean "!= RWSEM_UNLOCKED_VALUE". Right?

Uhhh ... I always get confused between assert and BUG_ON being opposite
polarity, but I think it's correct.

We are asserting that the rwsem is locked (either for read or write).
That is, it is a bug if the rwsem is unlocked.
So WARN_ON(sem->count == UNLOCKED_VALUE) is correct.  No?

> There are some inconsistency in the use of WARN_ON() and BUG_ON() in the
> assertions. For PREEMPT_RT, held_write is a BUG_ON. For non-PREEMPT_RT, held
> is a BUG_ON. It is not clear why one is BUG_ON and other one is WARN_ON. Is
> there a rationale for that?

I'll fix that up.

> BTW, we can actually check if the current process is the write-lock owner of
> a rwsem, but not for a reader-owned rwsem.

We actually don't want to do that.  See patches 3/4 where I explain how
XFS takes the XFS_ILOCK for write, then passes control to a workqueue
which asserts that the XFS_ILOCK is held for write.  The thread which
took the rwsem for write waits for the workqueue and unlocks the rwsem.