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Date: Fri, 15 Nov 2019 15:56:38 +0100 From: Jan Kara <jack@...e.cz> To: Sebastian Siewior <bigeasy@...utronix.de> Cc: Thomas Gleixner <tglx@...utronix.de>, Matthew Wilcox <willy@...radead.org>, LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Anna-Maria Gleixner <anna-maria@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>, Julia Cartwright <julia@...com>, Jan Kara <jack@...e.cz>, Theodore Tso <tytso@....edu>, Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>, Mark Fasheh <mark@...heh.com>, Joseph Qi <joseph.qi@...ux.alibaba.com>, Christoph Hellwig <hch@...radead.org>, Joel Becker <jlbec@...lplan.org> Subject: Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular spinlock_t On Fri 11-10-19 13:25:25, Sebastian Siewior wrote: > On 2019-08-20 20:01:14 [+0200], Thomas Gleixner wrote: > > On Tue, 20 Aug 2019, Matthew Wilcox wrote: > > > On Tue, Aug 20, 2019 at 07:08:18PM +0200, Sebastian Siewior wrote: > > > > Bit spinlocks are problematic if PREEMPT_RT is enabled, because they > > > > disable preemption, which is undesired for latency reasons and breaks when > > > > regular spinlocks are taken within the bit_spinlock locked region because > > > > regular spinlocks are converted to 'sleeping spinlocks' on RT. So RT > > > > replaces the bit spinlocks with regular spinlocks to avoid this problem. > > > > Bit spinlocks are also not covered by lock debugging, e.g. lockdep. > > > > > > > > Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock. > > > > > > > > Signed-off-by: Thomas Gleixner <tglx@...utronix.de> > > > > [bigeasy: remove the wrapper and use always spinlock_t] > > > > > > Uhh ... always grow the buffer_head, even for non-PREEMPT_RT? Why? > > > > Christoph requested that: > > > > https://lkml.kernel.org/r/20190802075612.GA20962@infradead.org > > What do we do about this one? I was thinking about this for quite some time. In the end I think the patch is almost fine but I'd name the lock b_update_lock and put it just after b_size element in struct buffer_head to use the hole there. That way we don't grow struct buffer_head. With some effort, we could even shrink struct buffer_head from 104 bytes (on x86_64) to 96 bytes but I don't think that effort is worth it (I'd find it better use of time to actually work on getting rid of buffer heads completely). Honza -- Jan Kara <jack@...e.com> SUSE Labs, CR
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