linux-kernel - Re: [patch 1/5] seqlock: Remove unused functions

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Date:	Thu, 15 Mar 2012 17:39:37 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [patch 1/5] seqlock: Remove unused functions

On Thu, Mar 15, 2012 at 09:29:50AM -0700, Linus Torvalds wrote:
> So I have to say, I hate this entire series.
> 
> Seriously, what the heck is the point of this churn? It's all entirely
> pointless searc-and-replace as far as I can tell, with absolutely zero
> upside.
> 
> It makes the low-level filesystems have to be aware of things that
> they don't want to know and *shouldn't* know. Why should a filesystem
> care that d_lock is a seqlock, and have to use a locking function that
> they've never seen before and is very specialized?
> 
> The "seq" part of the dentry is something only the lookup code and the
> internal dentry code should care about. NOBODY ELSE should ever care.

*nod*

There's another issue I have with that on API level, leaving aside any
questions of that being a good fit for dcache.  It's simply a bad interface:
we have variants that lock and play with d_seq, variants that play with
d_seq alone and, most commonly used, variant that locks but does not
touch d_seq at all.  IOW, we have traded "writes to d_seq must be under
d_lock" with "update-seq-without-locking primitive must be used after we'd
used lock-without-touching-seq one".  Which is not an improvement at all.
Sure, you can make a direct product out of anything; that doesn't make
the result a natural object.

The _only_ relationship between d_seq and d_lock is that the latter happens
to be serializing updates of the former.  For RT there's another one -
->d_lock taken to protect ->d_seq modifications really should not be
preempted in favour of anything that might do read_seqcount_begin on
->d_seq.  The biggest such section is in __d_move(), AFAICS, and it's not
_that_ big; can't RT simply have them protected by whatever it has that
really prevents preempt?

IOW, instead of all that stuff, how about
	about_to_modify_seq_holding_lock(&dentry->d_seq, &dentry->d_lock);
	done_modifying_seq(&dentry->d_seq, &dentry->d_lock);
around those 3 or 4 areas in fs/dcache.c, to give RT the missing information?
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