linux-kernel - Re: [PATCH] locking/rwsem: Remove unnessary check in rwsem_down_read

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Date:   Fri, 10 Nov 2023 14:54:07 +0800
From:   Tang Yizhou <yizhou.tang@...pee.com>
To:     Haifeng Xu <haifeng.xu@...pee.com>
Cc:     Waiman Long <longman@...hat.com>, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rwsem: Remove unnessary check in rwsem_down_read_slowpath()

On Thu, Nov 9, 2023 at 11:17 AM Haifeng Xu <haifeng.xu@...pee.com> wrote:
>
> reader          writer                                  reader
>
> acquire
> release
>                 rwsem_write_trylock
>                         set RWSEM_WRITER_LOCKED
>                                                         rwsem_down_read_slowpath
>                         set owner
>
> If prev lock holder is a reader, when it releases the lock, the owner isn't cleared(CONFIG_DEBUG_RWSEMS isn't enabled).
> A writer comes and can set the RWSEM_WRITER_LOCKED bit succsessfully, then a new reader run into slow path, before
> the writer set the owner, the new reader will see that both the RWSEM_READER_OWNED bit and RWSEM_WRITER_LOCKED bit are
> set.
>

For the above example, it won't cause a problem. When the writer
successfully sets RWSEM_WRITER_LOCKED, the reader, when reading rcnt
through rwsem_down_read_slowpath(), will see that rcnt is 0 and will
jump to the queue label.

Thanks,
Tang