linux-kernel - Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem

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Date:   Tue, 15 May 2018 13:33:03 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as
 non-spinnable in percpu_rwsem_release()

On 05/15, Jan Kara wrote:
>
> Now this behavior upsets lockdep and that's why we fool it by telling the
> semaphore got released before returning to userspace (through
> percpu_rwsem_release() helper) and similarly we tell lockdep we've got the
> semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now
> Amir has discovered that also rwsem debugging code gets confused by this
> behavior

Yes, plus someone else has already reported the problem a month ago,

> and previously also someone noticed that rwsem spinning does not
> make sense and can be broken by this behavior.

Well, this doesn't really matter but again, freeze_super() checks
frozen == SB_UNFROZEN under sb->s_umount and only then does sb_wait_write(),
when the previous writer has already realeased this lock. So the new writer
will never spin after lockdep_sb_freeze_release() clears ->owner.

Oleg.