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>

1 files changed, 6 insertions, 4 deletions

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ac677d032e3a..fe4a26d752d0 100644
--- 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);
         }