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
http://www.jlbec.org/
jlbec@xxxxxxxxxxxx
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