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Message-ID: <20190801175703.GH25064@quack2.suse.cz> Date: Thu, 1 Aug 2019 19:57:03 +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>, Theodore Tso <tytso@....edu>, Mark Fasheh <mark@...heh.com>, Joseph Qi <joseph.qi@...ux.alibaba.com>, Joel Becker <jlbec@...lplan.org>, linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org Subject: Re: [patch V2 6/7] fs/jbd2: Make state lock a spinlock On Thu 01-08-19 03:01:32, Thomas Gleixner wrote: > Bit-spinlocks are problematic on PREEMPT_RT if functions which might sleep > on RT, e.g. spin_lock(), alloc/free(), are invoked inside the lock held > region because bit spinlocks disable preemption even on RT. > > A first attempt was to replace state lock with a spinlock placed in struct > buffer_head and make the locking conditional on PREEMPT_RT and > DEBUG_BIT_SPINLOCKS. > > Jan pointed out that there is a 4 byte hole in struct journal_head where a > regular spinlock fits in and he would not object to convert the state lock > to a spinlock unconditionally. > > Aside of solving the RT problem, this also gains lockdep coverage for the > journal head state lock (bit-spinlocks are not covered by lockdep as it's > hard to fit a lockdep map into a single bit). > > The trivial change would have been to convert the jbd_*lock_bh_state() > inlines, but that comes with the downside that these functions take a > buffer head pointer which needs to be converted to a journal head pointer > which adds another level of indirection. > > As almost all functions which use this lock have a journal head pointer > readily available, it makes more sense to remove the lock helper inlines > and write out spin_*lock() at all call sites. > > Fixup all locking comments as well. > > Suggested-by: Jan Kara <jack@...e.com> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de> > Cc: "Theodore Ts'o" <tytso@....edu> > Cc: Mark Fasheh <mark@...heh.com> > Cc: Joseph Qi <joseph.qi@...ux.alibaba.com> > Cc: Joel Becker <jlbec@...lplan.org> > Cc: Jan Kara <jack@...e.com> > Cc: linux-ext4@...r.kernel.org Just a heads up that I didn't miss this patch. Just it has some bugs and I figured that rather than explaining to you subtleties of jh lifetime it is easier to fix up the problems myself since you're probably not keen on becoming jbd2 developer ;)... which was more complex than I thought so I'm not completely done yet. Hopefuly tomorrow. Honza -- Jan Kara <jack@...e.com> SUSE Labs, CR
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