From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
linux-rt-users <linux-rt-users@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Clark Williams <williams@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
jack@suse.com, Waiman Long <longman@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)
Date: Mon, 6 May 2019 18:50:09 +0200 [thread overview]
Message-ID: <20190506165009.GA28959@redhat.com> (raw)
In-Reply-To: <20190503145059.GC2606@hirez.programming.kicks-ass.net>
On 05/03, Peter Zijlstra wrote:
>
> -static void lockdep_sb_freeze_release(struct super_block *sb)
> -{
> - int level;
> -
> - for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
> - percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> -}
> -
> -/*
> - * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
> - */
> -static void lockdep_sb_freeze_acquire(struct super_block *sb)
> -{
> - int level;
> -
> - for (level = 0; level < SB_FREEZE_LEVELS; ++level)
> - percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> + percpu_down_write_non_owner(sb->s_writers.rw_sem + level-1);
> }
I'd suggest to not change fs/super.c, keep these helpers, and even not introduce
xxx_write_non_owner().
freeze_super() takes other locks, it calls sync_filesystem(), freeze_fs(), lockdep
should know that this task holds SB_FREEZE_XXX locks for writing.
> @@ -80,14 +83,8 @@ int __percpu_down_read(struct percpu_rw_
> * and reschedule on the preempt_enable() in percpu_down_read().
> */
> preempt_enable_no_resched();
> -
> - /*
> - * Avoid lockdep for the down/up_read() we already have them.
> - */
> - __down_read(&sem->rw_sem);
> + wait_event(sem->waiters, !atomic_read(&sem->block));
> this_cpu_inc(*sem->read_count);
Argh, this looks racy :/
Suppose that sem->block == 0 when wait_event() is called, iow the writer released
the lock.
Now suppose that this __percpu_down_read() races with another percpu_down_write().
The new writer can set sem->block == 1 and call readers_active_check() in between,
after wait_event() and before this_cpu_inc(*sem->read_count).
Oleg.
next prev parent reply other threads:[~2019-05-06 16:50 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-26 9:34 [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16) Juri Lelli
2019-03-28 10:17 ` Sebastian Andrzej Siewior
2019-04-19 8:56 ` Juri Lelli
2019-04-30 12:51 ` Sebastian Andrzej Siewior
2019-04-30 13:28 ` Peter Zijlstra
2019-04-30 13:45 ` Sebastian Andrzej Siewior
2019-04-30 14:01 ` Peter Zijlstra
2019-04-30 14:15 ` Oleg Nesterov
2019-04-30 14:29 ` Peter Zijlstra
2019-04-30 14:42 ` Oleg Nesterov
2019-04-30 14:44 ` Peter Zijlstra
2019-04-30 14:53 ` Oleg Nesterov
2019-05-01 17:09 ` Peter Zijlstra
2019-05-01 17:26 ` Waiman Long
2019-05-01 18:54 ` Peter Zijlstra
2019-05-01 19:22 ` Davidlohr Bueso
2019-05-01 19:25 ` Peter Zijlstra
2019-05-02 10:09 ` Oleg Nesterov
2019-05-02 11:42 ` Oleg Nesterov
2019-05-03 14:50 ` Peter Zijlstra
2019-05-03 15:25 ` Peter Zijlstra
2019-05-06 16:50 ` Oleg Nesterov [this message]
2019-06-19 9:50 ` Peter Zijlstra
2019-05-03 14:16 ` Peter Zijlstra
2019-05-03 15:37 ` Oleg Nesterov
2019-05-03 15:46 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190506165009.GA28959@redhat.com \
--to=oleg@redhat.com \
--cc=bigeasy@linutronix.de \
--cc=bristot@redhat.com \
--cc=dave@stgolabs.net \
--cc=jack@suse.com \
--cc=juri.lelli@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rt-users@vger.kernel.org \
--cc=longman@redhat.com \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=williams@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).