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 20:17:32 -0500
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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 11/14/23 16:26, Matthew Wilcox wrote:
> 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?
You are right. I got confused too.
>
>> 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.
The check for write lock ownership is accurate. OTOH, the locked check 
can have false positive and so is less reliable.
>
>> 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.
>
I see. Thanks for the explanation.

Cheers,
Longman