Re: [PATCH] fsstack: fsstack_copy_inode_size locking

[Christoph Hellwig <hch@infradead.org>]

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_*).