From: Oleg Nesterov <oleg@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>,
Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
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>
Subject: Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()
Date: Tue, 15 May 2018 13:33:03 +0200 [thread overview]
Message-ID: <20180515113303.GB14527@redhat.com> (raw)
In-Reply-To: <20180515090052.cvlt3e7ue6bm4c2c@quack2.suse.cz>
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.
next prev parent reply other threads:[~2018-05-15 11:33 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-14 19:31 [RFC PATCH v2 0/2] locking/rwsem: Fix DEBUG_RWSEM warning from thaw_sup Waiman Long
2018-05-14 19:31 ` [RFC PATCH v2 1/2] locking/rwsem: Add a new RWSEM_WRITER_OWNED_NOSPIN flag Waiman Long
2018-05-15 6:59 ` Amir Goldstein
2018-05-15 8:25 ` Peter Zijlstra
2018-05-14 19:31 ` [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release() Waiman Long
2018-05-15 5:42 ` Amir Goldstein
2018-05-15 7:04 ` Amir Goldstein
2018-05-15 13:45 ` Waiman Long
2018-05-15 8:35 ` Peter Zijlstra
2018-05-15 9:00 ` Jan Kara
2018-05-15 11:33 ` Oleg Nesterov [this message]
2018-05-15 8:51 ` Peter Zijlstra
2018-05-15 11:06 ` Oleg Nesterov
2018-05-15 11:51 ` Peter Zijlstra
2018-05-15 12:45 ` Oleg Nesterov
2018-05-15 12:58 ` Peter Zijlstra
2018-05-15 13:57 ` Waiman Long
2018-05-15 14:00 ` Matthew Wilcox
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