NTFS: Add fs/ntfs/mft.c::try_map_mft_record() which fails with -EALREADY if

      the mft record is already locked and otherwise behaves the same way
      as fs/ntfs/mft.c::map_mft_record().
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>

fs/ntfs/ChangeLog

@@ -70,6 +70,9 @@ ToDo/Notes:
 	  mft record dirty as well as the page itself.
 	- Fix compiler warnings on x86-64 in fs/ntfs/dir.c.  (Randy Dunlap,
 	  slightly modified by me)
+	- Add fs/ntfs/mft.c::try_map_mft_record() which fails with -EALREADY if
+	  the mft record is already locked and otherwise behaves the same way
+	  as fs/ntfs/mft.c::map_mft_record().
 
 2.1.20 - Fix two stupid bugs introduced in 2.1.18 release.
 

fs/ntfs/mft.c

@@ -114,6 +114,57 @@ static inline MFT_RECORD *map_mft_record_page(ntfs_inode *ni)
 	return (void*)page;
 }
 
+/**
+ * try_map_mft_record - attempt to map, pin and lock an mft record
+ * @ni:		ntfs inode whose MFT record to map
+ *
+ * First, attempt to take the mrec_lock semaphore.  If the semaphore is already
+ * taken by someone else, return the error code -EALREADY.  Otherwise continue
+ * as described below.
+ *
+ * The page of the record is mapped using map_mft_record_page() before being
+ * returned to the caller.
+ *
+ * This in turn uses ntfs_map_page() to get the page containing the wanted mft
+ * record (it in turn calls read_cache_page() which reads it in from disk if
+ * necessary, increments the use count on the page so that it cannot disappear
+ * under us and returns a reference to the page cache page).
+ *
+ * The mft record is now ours and we return a pointer to it.  You need to check
+ * the returned pointer with IS_ERR() and if that is true, PTR_ERR() will return
+ * the error code.
+ *
+ * For further details see the description of map_mft_record() below.
+ */
+MFT_RECORD *try_map_mft_record(ntfs_inode *ni)
+{
+	MFT_RECORD *m;
+
+	ntfs_debug("Entering for mft_no 0x%lx.", ni->mft_no);
+
+	/* Make sure the ntfs inode doesn't go away. */
+	atomic_inc(&ni->count);
+
+	/*
+	 * Serialize access to this mft record.  If someone else is already
+	 * holding the lock, abort instead of waiting for the lock.
+	 */
+	if (unlikely(down_trylock(&ni->mrec_lock))) {
+		ntfs_debug("Mft record is already locked, aborting.");
+		atomic_dec(&ni->count);
+		return ERR_PTR(-EALREADY);
+	}
+
+	m = map_mft_record_page(ni);
+	if (likely(!IS_ERR(m)))
+		return m;
+
+	up(&ni->mrec_lock);
+	atomic_dec(&ni->count);
+	ntfs_error(ni->vol->sb, "Failed with error code %lu.", -PTR_ERR(m));
+	return m;
+}
+
 /**
  * map_mft_record - map, pin and lock an mft record
  * @ni:		ntfs inode whose MFT record to map

fs/ntfs/mft.h

@@ -29,6 +29,7 @@
 
 #include "inode.h"
 
+extern MFT_RECORD *try_map_mft_record(ntfs_inode *ni);
 extern MFT_RECORD *map_mft_record(ntfs_inode *ni);
 extern void unmap_mft_record(ntfs_inode *ni);