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

[Andrew Morton <akpm@zip.com.au>]

Christoph Hellwig wrote:
> 
> 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.

It could be time to separate out a __generic_file_write() which
doesn't take i_sem at all.  The ext3 tree was doing that for a
while, to permit multipage transactions in journalled data mode.

> 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..

hm.  Are you sure about that?  They look fairly useful to me?

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