On Thu, Dec 18, 2008 at 02:12:34PM +0100, Jan Kara wrote:

  This is the thing I was wondering about. Why exactly is the spinlock
necessary for blkdev_releasepage()? I understand we have to protect
reading client_releasepage() pointer because it could change but my point
was that it changes only during mount / umount.

Hmm.... I suppose we could use RCU, but then we'd have to worry about
the race condition where client_releasepage() gets called after the
umount has happened.

quoted

I also think we are sad that we cannot implement various
implementations for client_releasepage(). But now I cannot imagine
what to do for a client_releasepage() which can sleep, too...

My suggestion is that we not worry about making changes to
fs/block_dev.c to allow client_releasepage() to sleep until we have
filesystems that really need client_releasepage() to sleep.  It
probably is possible, with appropriate atomic bit sets for flags to
indicate an unmount in progress, and client_releasepage in progress,
and use of RCU, we could allow client_releasepage.  But it might not
be worth it unless there is a filesystem that really needs it.

   	    	   	      		      	  - Ted