linux-kernel - Re: [PATCH v3 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY

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Message-ID: <20180515175514.GM12217@hirez.programming.kicks-ass.net>
Date:   Tue, 15 May 2018 19:55:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     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>,
        Oleg Nesterov <oleg@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED
 flag

On Tue, May 15, 2018 at 01:52:06PM -0400, Waiman Long wrote:
> On 05/15/2018 01:48 PM, Peter Zijlstra wrote:
> > On Tue, May 15, 2018 at 01:38:03PM -0400, Waiman Long wrote:
> >> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> >> index 30465a2..b7208e1 100644
> >> --- a/kernel/locking/rwsem.c
> >> +++ b/kernel/locking/rwsem.c
> >> @@ -130,7 +130,8 @@ void up_read(struct rw_semaphore *sem)
> >>  void up_write(struct rw_semaphore *sem)
> >>  {
> >>  	rwsem_release(&sem->dep_map, 1, _RET_IP_);
> >> -	DEBUG_RWSEMS_WARN_ON(sem->owner != current);
> >> +	DEBUG_RWSEMS_WARN_ON((sem->owner != current) &&
> >> +			     !rwsem_has_anonymous_owner(sem->owner));
> > Why? Don't we always do percpu_rwsem_acquire() before up?
> >
> 
> This is to allow an unlock to happen in the unknown owner state. Yes, it
> is not necessary to fix the percpu-rwsem problem. It is there just in
> case a rwsem will be released in that state in the future.

Let's not allow that until there's a very good reason for it.