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

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Message-ID: <da1daacf-22b2-20da-b6c0-9b1362b8a901@redhat.com>
Date:   Thu, 7 Sep 2023 17:06:39 -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 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.

Cheers,
Longman