Re: [patch 11/13] don't hold i_sem during O_DIRECT writes to blockdevs

[Christoph Hellwig <hch@infradead.org>]

On Sun, Jul 28, 2002 at 12:33:23AM -0700, Andrew Morton wrote:
> This patch changes O_DIRECT writes to blockdevs so that they no longer
> run under i_sem.

Please don't make this depenend on S_ISBLK().  There are filesystems (like
XFS) that are designed to safely allow concurrent O_DIRECT writes to
regular files.

Toe implement this properly we should drop i_sem in the ->direct_IO method
of the filesystem/blockdevice.  The only question remaining is whether the
method has to reqacquire it before returing (and it'll be imediately
released again or whether we should change semantics of ->direct_IO to
always drop the lock.

The third options would be to never call ->direct_IO with the i_sem held
and let filesystems that need it (only ext2 in 2.5 mainline) do
synchronization themselves.

I think I prefer option 3, it's the cleanest way of doing it.

A little unrelated, but as you touch the code:  what about removing the two
existing special cases for S_ISBLK() in generic_file_write()?  they're
present only to provide the old (pre-LFS) blockdevice semantics on 2.4,
we shouldn't keept them around forever..