Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()

[Waiman Long <longman@redhat.com>]

On 9/7/23 17:06, Waiman Long wrote:
>
> On 9/7/23 15:33, Matthew Wilcox wrote:
>> On Thu, Sep 07, 2023 at 02:05:54PM -0400, Waiman Long wrote:
>>> On 9/7/23 13:47, Matthew Wilcox (Oracle) wrote:
>>>> +static inline int rwsem_is_write_locked(struct rw_semaphore *sem)
>>>> +{
>>>> +    return atomic_long_read(&sem->count) & 1 /* 
>>>> RWSEM_WRITER_LOCKED */;
>>>> +}
>>> I would prefer you move the various RWSEM_* count bit macros from
>>> kernel/locking/rwsem.c to under the !PREEMPT_RT block and directly use
>>> RWSEM_WRITER_LOCKED instead of hardcoding a value of 1.
>> Just to be clear, you want the ~50 lines from:
>>
>> /*
>>   * On 64-bit architectures, the bit definitions of the count are:
>> ...
>> #define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
>> RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
>>
>> moved from rwsem.c to rwsem.h?
>>
>> Or just these four lines:
>>
>> #define RWSEM_WRITER_LOCKED     (1UL << 0)
>> #define RWSEM_FLAG_WAITERS      (1UL << 1)
>> #define RWSEM_FLAG_HANDOFF      (1UL << 2)
>> #define RWSEM_FLAG_READFAIL     (1UL << (BITS_PER_LONG - 1))
>
> I think just the first 3 lines will be enough. Maybe a bit of comment 
> about these bit flags in the count atomic_long value.

Actually, the old rwsem implementation won't allow you to reliably 
determine if a rwsem is write locked because the xadd instruction is 
used for write locking and the code had to back out the WRITER_BIAS if 
the attempt failed. Maybe that is why XFS has its own code to check if a 
rwsem is write locked which is needed with the old rwsem implementation.

The new implementation makes this check reliable. Still it is not easy 
to check if a rwsem is read locked as the check will be rather 
complicated and probably racy.

Cheers,
Longman