linux-kernel - Re: [PATCH 1/5] locking: Add rwsem_is_write

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Message-ID: <c38847cb-92c9-139f-03cc-86d233297d58@redhat.com>
Date:   Thu, 7 Sep 2023 19:47:31 -0400
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
Subject: Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()


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