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SubjectRe: [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;
>>> }
>>> +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.


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