xfs: take the ilock shared if possible in xfs_file_iomap_begin

We always just read the extent first, and will later lock exlusively
after first dropping the lock in case we actually allocate blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ac677d032e3a95be827cda69d2694fe5e39637f8..fe4a26d752d0000597a8453e532882c4862259b3 100644 (file)
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -954,6 +954,7 @@ xfs_file_iomap_begin(
        struct xfs_bmbt_irec    imap;
        xfs_fileoff_t           offset_fsb, end_fsb;
        int                     nimaps = 1, error = 0;
+       unsigned                lockmode;
 
        if (XFS_FORCED_SHUTDOWN(mp))
                return -EIO;
@@ -963,7 +964,7 @@ xfs_file_iomap_begin(
                                iomap);
        }
 
-       xfs_ilock(ip, XFS_ILOCK_EXCL);
+       lockmode = xfs_ilock_data_map_shared(ip);
 
        ASSERT(offset <= mp->m_super->s_maxbytes);
        if ((xfs_fsize_t)offset + length > mp->m_super->s_maxbytes)
@@ -974,7 +975,7 @@ xfs_file_iomap_begin(
        error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
                               &nimaps, XFS_BMAPI_ENTIRE);
        if (error) {
-               xfs_iunlock(ip, XFS_ILOCK_EXCL);
+               xfs_iunlock(ip, lockmode);
                return error;
        }
 
@@ -994,7 +995,8 @@ xfs_file_iomap_begin(
                 * xfs_iomap_write_direct() expects the shared lock. It
                 * is unlocked on return.
                 */
-               xfs_ilock_demote(ip, XFS_ILOCK_EXCL);
+               if (lockmode == XFS_ILOCK_EXCL)
+                       xfs_ilock_demote(ip, lockmode);
                error = xfs_iomap_write_direct(ip, offset, length, &imap,
                                nimaps);
                if (error)
@@ -1005,7 +1007,7 @@ xfs_file_iomap_begin(
        } else {
                ASSERT(nimaps);
 
-               xfs_iunlock(ip, XFS_ILOCK_EXCL);
+               xfs_iunlock(ip, lockmode);
                trace_xfs_iomap_found(ip, offset, length, 0, &imap);
        }