linux-kernel - Re: [PATCH] fsstack: fsstack_copy_inode

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Message-ID: <20080629071528.GA31114@infradead.org>
Date:	Sun, 29 Jun 2008 03:15:28 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Erez Zadok <ezk@...sunysb.edu>,
	Andrew Morton <akpm@...ux-foundation.org>, mhalcrow@...ibm.com,
	hooanon05@...oo.co.jp, hch@...radead.org, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fsstack: fsstack_copy_inode_size locking

On Sun, Jun 29, 2008 at 01:05:09AM +0100, Hugh Dickins wrote:
> LTP's iogen01 doio tests used to hang nicely on 32-bit SMP when /tmp was a
> unionfs mount of a tmpfs, i_size_read spinning forever, waiting for a lost
> seqcount update: fixed by taking i_lock around i_size_write when 32-bit SMP.
> 
> But akpm was dissatisfied with the resulting patch: its lack of commentary,
> the #ifs, the nesting around i_size_read, the lack of attention to i_blocks.
> I promised to redo it with the general spin_lock_32bit() he proposed; but 
> disliked the result, partly because "32bit" obscures the real constraints,
> which are best commented within fsstack_copy_inode_size itself.
> 
> This version adds those comments, and uses sizeof comparisons which the
> compiler can optimize out, instead of CONFIG_SMP, CONFIG_LSF. BITS_PER_LONG.

Btw, I hope fsstack doesn't rely on i_size having any particular
meaning.  As far as the VFS is concerned i_size is field only used by
the filesystem (or library routines like generic_file_*).

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