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 <h...@lst.de>
---
 fs/xfs/xfs_iomap.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 5d06a2d..c3cc175 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -955,6 +955,7 @@ 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;
@@ -964,7 +965,7 @@ 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)
@@ -975,7 +976,7 @@ 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;
        }
 
@@ -995,7 +996,8 @@ 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)
@@ -1006,7 +1008,7 @@ 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);
        }
 
-- 
2.1.4

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