Re: [PATCH RFC] O_DIRECT reads and writes without i_sem

[Christoph Hellwig <hch@infradead.org>]

On Mon, Nov 01, 2004 at 10:32:07AM -0500, Chris Mason wrote:
> Hello everyone,
> 
> Right now, O_DIRECT reads and writes on regular files have to take i_sem
> while reading file metadata in order to make sure we don't race with
> hole filling.
> 
> This patch tries to get around that by avoiding i_sem when we are doing
> an O_DIRECT read or write inside of i_size.  Yet another rw semaphore is
> added to struct inode to protect against holes being filled during the
> O_DIRECT.  direct-io.c gets another special case to be aware of the
> locking.
> 
> This has only been lightly tested, I'm posting here for general comments
> before I go much further.  I'm rounding up some hardware with enough
> disks to benchmark it properly.  
> 
> Only ext2 and reiserfs are modified to drop i_sem during O_DIRECT. ext3
> needs some care around the orphan lists, and I didn't want to get into
> that until the rest of the patch was working.

This gets too complicated for it's own sake.  What about going down the
XFS route and making i_sem a r/w semaphore that's taken only shared
during read and write I/O, but exclusive while setting up write I/O
outside of i_size?  Alternatively just move the I/O locking into the
filesytem.