On Mon, Jun 20, 2011 at 04:15:37PM -0400, Christoph Hellwig wrote:

i_alloc_sem is a rather special rw_semaphore.  It's the last one that may
be released by a non-owner, and it's write side is always mirrored by
real exclusion.  It's intended use it to wait for all pending direct I/O
requests to finish before starting a truncate.

Replace it with a hand-grown construct:

 - exclusion for truncates is already guaranteed by i_mutex, so it can
   simply fall way
 - the reader side is replaced by an i_dio_count member in struct inode
   that counts the number of pending direct I/O requests.  Truncate can't
   proceed as long as it's non-zero
 - when i_dio_count reaches non-zero we wake up a pending truncate using
   wake_up_bit on a new bit in i_flags
 - new references to i_dio_count can't appear while we are waiting for
   it to read zero because the direct I/O count always needs i_mutex
   (or an equivalent like XFS's i_iolock) for starting a new operation.

This scheme is much simpler, and saves the space of a spinlock_t and a
struct list_head in struct inode (typically 160 bytes on a non-debug 64-bit
system).

	Are we guaranteed that all allocation changes are locked out by
i_dio_count>0?  I don't think we are.  The ocfs2 code very strongly
assumes the state of a file's allocation when it holds i_alloc_sem.  I
feel like we lose that here. 

Joel

-- 

"I don't even butter my bread; I consider that cooking."
         - Katherine Cebrian

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