From: Jan Kara <jack@suse.cz>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
Sebastian Siewior <bigeasy@linutronix.de>,
Anna-Maria Gleixner <anna-maria@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Julia Cartwright <julia@ni.com>, Jan Kara <jack@suse.com>,
linux-ext4@vger.kernel.org, Theodore Tso <tytso@mit.edu>,
Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, Mark Fasheh <mark@fasheh.com>,
Joseph Qi <joseph.qi@linux.alibaba.com>,
Joel Becker <jlbec@evilplan.org>
Subject: Re: [patch V2 7/7] fs/jbd2: Free journal head outside of locked region
Date: Thu, 1 Aug 2019 11:22:23 +0200 [thread overview]
Message-ID: <20190801092223.GG25064@quack2.suse.cz> (raw)
In-Reply-To: <20190801010944.549462805@linutronix.de>
On Thu 01-08-19 03:01:33, Thomas Gleixner wrote:
> On PREEMPT_RT bit-spinlocks have the same semantics as on PREEMPT_RT=n,
> i.e. they disable preemption. That means functions which are not safe to be
> called in preempt disabled context on RT trigger a might_sleep() assert.
>
> The journal head bit spinlock is mostly held for short code sequences with
> trivial RT safe functionality, except for one place:
>
> jbd2_journal_put_journal_head() invokes __journal_remove_journal_head()
> with the journal head bit spinlock held. __journal_remove_journal_head()
> invokes kmem_cache_free() which must not be called with preemption disabled
> on RT.
>
> Jan suggested to rework the removal function so the actual free happens
> outside the bit-spinlocked region.
>
> Split it into two parts:
>
> - Do the sanity checks and the buffer head detach under the lock
>
> - Do the actual free after dropping the lock
>
> There is error case handling in the free part which needs to dereference
> the b_size field of the now detached buffer head. Due to paranoia (caused
> by ignorance) the size is retrieved in the detach function and handed into
> the free function. Might be over-engineered, but better safe than sorry.
>
> This makes the journal head bit-spinlock usage RT compliant and also avoids
> nested locking which is not covered by lockdep.
>
> Suggested-by: Jan Kara <jack@suse.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: linux-ext4@vger.kernel.org
> Cc: "Theodore Ts'o" <tytso@mit.edu>
> Cc: Jan Kara <jack@suse.com>
Looks mostly good. Just a small suggestion for simplification below:
> @@ -2559,11 +2568,14 @@ void jbd2_journal_put_journal_head(struc
> J_ASSERT_JH(jh, jh->b_jcount > 0);
> --jh->b_jcount;
> if (!jh->b_jcount) {
> - __journal_remove_journal_head(bh);
> + size_t b_size = __journal_remove_journal_head(bh);
> +
> jbd_unlock_bh_journal_head(bh);
> + journal_release_journal_head(jh, b_size);
> __brelse(bh);
The bh is pinned until you call __brelse(bh) above and bh->b_size doesn't
change during the lifetime of the buffer. So there's no need of
fetching bh->b_size in __journal_remove_journal_head() and passing it back.
You can just:
journal_release_journal_head(jh, bh->b_size);
> - } else
> + } else {
> jbd_unlock_bh_journal_head(bh);
> + }
> }
>
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-08-01 16:55 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 1:01 [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging Thomas Gleixner
2019-08-01 1:01 ` [patch V2 1/7] locking/lockdep: Add Kconfig option for bit spinlocks Thomas Gleixner
2019-08-01 1:01 ` [patch V2 2/7] fs/buffer: Move BH_Uptodate_Lock locking into wrapper functions Thomas Gleixner
2019-08-01 1:01 ` [patch V2 3/7] fs/buffer: Substitute BH_Uptodate_Lock for RT and bit spinlock debugging Thomas Gleixner
2019-08-01 1:01 ` [patch V2 4/7] fs/jbd2: Remove jbd_trylock_bh_state() Thomas Gleixner
2019-08-01 9:00 ` Jan Kara
2019-08-01 1:01 ` [patch V2 5/7] fs/jbd2: Simplify journal_unmap_buffer() Thomas Gleixner
2019-08-01 9:04 ` Jan Kara
2019-08-01 1:01 ` [patch V2 6/7] fs/jbd2: Make state lock a spinlock Thomas Gleixner
2019-08-01 11:28 ` Peter Zijlstra
2019-08-02 13:31 ` Jan Kara
2019-08-01 17:57 ` Jan Kara
2019-08-01 18:12 ` Thomas Gleixner
2019-08-02 13:37 ` Jan Kara
2019-08-02 15:29 ` Thomas Gleixner
2019-08-01 1:01 ` [patch V2 7/7] fs/jbd2: Free journal head outside of locked region Thomas Gleixner
2019-08-01 9:22 ` Jan Kara [this message]
2019-08-01 18:08 ` Thomas Gleixner
2019-08-02 7:56 ` [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging Christoph Hellwig
2019-08-02 9:07 ` Thomas Gleixner
2019-08-06 6:11 ` Christoph Hellwig
2019-08-08 7:02 ` Thomas Gleixner
2019-08-08 7:28 ` Christoph Hellwig
2019-08-08 7:54 ` Thomas Gleixner
2019-08-10 8:18 ` Christoph Hellwig
2019-08-11 1:22 ` Matthew Wilcox
2019-08-20 17:16 ` Sebastian Siewior
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