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: Thu, 2 May 2019 12:09:32 +0200 [thread overview]
Message-ID: <20190502100932.GA7323@redhat.com> (raw)
In-Reply-To: <20190501170953.GB2650@hirez.programming.kicks-ass.net>
On 05/01, Peter Zijlstra wrote:
>
> Anyway; I cobbled together the below. Oleg, could you have a look, I'm
> sure I messed it up.
Oh, I will need to read this carefully. but at first glance I do not see
any hole...
> +static void readers_block(struct percpu_rw_semaphore *sem)
> +{
> + wait_event_cmd(sem->writer, !sem->readers_block,
> + __up_read(&sem->rw_sem), __down_read(&sem->rw_sem));
> +}
> +
> +static void block_readers(struct percpu_rw_semaphore *sem)
> +{
> + wait_event_exclusive_cmd(sem->writer, !sem->readers_block,
> + __up_write(&sem->rw_sem),
> + __down_write(&sem->rw_sem));
> + /*
> + * Notify new readers to block; up until now, and thus throughout the
> + * longish rcu_sync_enter() above, new readers could still come in.
> + */
> + WRITE_ONCE(sem->readers_block, 1);
> +}
So iiuc, despite it name block_readers() also serializes the writers, ->rw_sem
can be dropped by down_write_non_owner() so the new writer can take this lock.
And note that the caller of readers_block() does down_read(), the caller of
block_readers() does down_write(). So perhaps it makes sense to shift these
down_read/write into the helpers above and rename them,
void xxx_down_read(struct percpu_rw_semaphore *sem)
{
__down_read(&sem->rw_sem);
wait_event_cmd(sem->writer, !sem->readers_block,
__up_read(&sem->rw_sem), __down_read(&sem->rw_sem));
}
void xxx_down_write(struct percpu_rw_semaphore *sem)
{
down_write(&sem->rw_sem);
wait_event_exclusive_cmd(sem->writer, !sem->readers_block,
__up_write(&sem->rw_sem),
__down_write(&sem->rw_sem));
/*
* Notify new readers to block; up until now, and thus throughout the
* longish rcu_sync_enter() above, new readers could still come in.
*/
WRITE_ONCE(sem->readers_block, 1);
}
to make this logic more clear? Or even
bool ck_read(struct percpu_rw_semaphore *sem)
{
__down_read(&sem->rw_sem);
if (!sem->readers_block)
return true;
__up_read(&sem->rw_sem);
}
bool ck_write(struct percpu_rw_semaphore *sem)
{
down_write(&sem->rw_sem);
if (!sem->readers_block)
return true;
up_write(&sem->rw_sem);
}
Then percpu_down_read/write can simply do wait_event(ck_read(sem)) and
wait_event_exclusive(ck_write(sem)) respectively.
But this all is cosmetic, it seems that we can remove ->rw_sem altogether
but I am not sure...
Oleg.
next prev parent reply other threads:[~2019-05-02 10:09 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 [this message]
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
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=20190502100932.GA7323@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).