Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: 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>, Jan Kara <jack@suse.cz>
Subject: Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()
Date: Tue, 15 May 2018 13:51:12 +0200	[thread overview]
Message-ID: <20180515115112.GZ12217@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180515110632.GA14527@redhat.com>

On Tue, May 15, 2018 at 01:06:33PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > So what's wrong with adding:
> >
> > 	if (!read)
> > 		sem->rw_sem.owner = current;
> 
> Agreed, I have already suggested this change twice. Except we obviously
> need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
> or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

Right, details ;-)

> > Afaict the whole .owner=NULL thing in release already stops the spinners
> 
> Not really, the new writer will spin in this case, afaics.
> 
> But this is another problem and probably we do not care. The new writer is
> almost impossible in this particular case, another freeze_super() should
> notice frozen != SB_UNFROZEN and return EBUSY.

rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
&& owner != RWSEM_READER_OWNED, which will fail for !owner.

Or am I completely confused again?

> > and the above 'fixes' the debug splat.
> 
> Yes.
> 
> Waiman, can't we trivially fix the problem first? Then we can add the helpers
> and think about other improvements.

It is really simple; we're not going to add public (and EXPORT'ed to
boot) interfaces to rwsem for this.