From: Oleg Nesterov <oleg@redhat.com>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Davidlohr Bueso <dave@stgolabs.net>,
"Theodore Y. Ts'o" <tytso@mit.edu>,
Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.cz>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED flag
Date: Wed, 16 May 2018 12:48:30 +0200 [thread overview]
Message-ID: <20180516104829.GA24332@redhat.com> (raw)
In-Reply-To: <1526420991-21213-2-git-send-email-longman@redhat.com>
On 05/15, Waiman Long wrote:
>
> There are use cases where a rwsem can be acquired by one task, but
> released by another task. In thess cases, optimistic spinning may need
> to be disabled. One example will be the filesystem freeze/thaw code
You do not read my emails ;)
Let me repeat once again that in this particular case the writer will
never spin because of owner == NULL. freeze_super() checks SB_UNFROZEN
under sb->s_umount and only then calls sb_wait_write(). IOW, sb_wait_write()
can only be called when this rwsem was already released by the previous
writer.
I am not arguing with this change, percpu_rwsem_release/acquire may have
another user sometime, but the changelog is not accurate.
> +static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
> {
> - return owner && owner != RWSEM_READER_OWNED;
> + return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
> }
Perhaps you should add __attribute__(aligned) to struct rw_semaphore then...
I don't think it is really needed, but see the comment under struct address_space.
Oleg.
next prev parent reply other threads:[~2018-05-16 10:48 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-15 21:49 [PATCH v4 0/2] locking/rwsem: Fix DEBUG_RWSEM warning from thaw_super() Waiman Long
2018-05-15 21:49 ` [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED flag Waiman Long
2018-05-16 10:48 ` Oleg Nesterov [this message]
2018-05-16 11:59 ` Peter Zijlstra
2018-05-16 13:11 ` Waiman Long
2018-05-16 15:27 ` Oleg Nesterov
2018-05-16 12:19 ` Matthew Wilcox
2018-05-18 7:02 ` Ingo Molnar
2018-05-18 8:41 ` Oleg Nesterov
2018-05-18 9:40 ` Ingo Molnar
2018-05-18 16:55 ` [PATCH] locking/rwsem: simplify the is-owner-spinnable checks Oleg Nesterov
2018-05-18 17:00 ` Waiman Long
2018-05-15 21:49 ` [PATCH v4 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN Waiman Long
2018-05-16 5:37 ` Amir Goldstein
2018-05-16 13:17 ` Waiman Long
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