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Message-ID: <20190801092223.GG25064@quack2.suse.cz> Date: Thu, 1 Aug 2019 11:22:23 +0200 From: Jan Kara <jack@...e.cz> To: Thomas Gleixner <tglx@...utronix.de> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Sebastian Siewior <bigeasy@...utronix.de>, Anna-Maria Gleixner <anna-maria@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>, Julia Cartwright <julia@...com>, Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org, Theodore Tso <tytso@....edu>, Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org, Mark Fasheh <mark@...heh.com>, Joseph Qi <joseph.qi@...ux.alibaba.com>, Joel Becker <jlbec@...lplan.org> Subject: Re: [patch V2 7/7] fs/jbd2: Free journal head outside of locked region 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@...e.com> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de> > Cc: linux-ext4@...r.kernel.org > Cc: "Theodore Ts'o" <tytso@....edu> > Cc: Jan Kara <jack@...e.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@...e.com> SUSE Labs, CR
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