sysfs: slim down sysfs_dirent->s_active

Make sysfs_dirent->s_active an atomic_t instead of rwsem.  This
reduces the size of sysfs_dirent from 136 to 104 on 64bit and from 76
to 60 on 32bit with lock debugging turned off.  With lock debugging
turned on the reduction is much larger.

s_active starts at zero and each active reference increments s_active.
Putting a reference decrements s_active.  Deactivation subtracts
SD_DEACTIVATED_BIAS which is currently INT_MIN and assumed to be small
enough to make s_active negative.  If s_active is negative,
sysfs_get() no longer grants new references.  Deactivation succeeds
immediately if there is no active user; otherwise, it waits using a
completion for the last put.

Due to the removal of lockdep tricks, this change makes things less
trickier in release_sysfs_dirent().  As all the complexity is
contained in three s_active functions, I think it's more readable this
way.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 2 insertions, 11 deletions

diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index f8779eaa53ff..ae006b070bf0 100644
--- a/fs/sysfs/sysfs.h
+++ b/fs/sysfs/sysfs.h
@@ -21,7 +21,7 @@ struct sysfs_elem_bin_attr {
  */
 struct sysfs_dirent {
         atomic_t                s_count;
-        struct rw_semaphore     s_active;
+        atomic_t                s_active;
         struct sysfs_dirent     * s_parent;
         struct list_head        s_sibling;
         struct list_head        s_children;
@@ -42,16 +42,7 @@ struct sysfs_dirent {
         atomic_t                s_event;
 };
 
-/*
- * A sysfs file which deletes another file when written to need to
- * write lock the s_active of the victim while its s_active is read
- * locked for the write operation.  Tell lockdep that this is okay.
- */
-enum sysfs_s_active_class
-{
-        SYSFS_S_ACTIVE_NORMAL,          /* file r/w access, etc - default */
-        SYSFS_S_ACTIVE_DEACTIVATE,      /* file deactivation */
-};
+#define SD_DEACTIVATED_BIAS     INT_MIN
 
 extern struct vfsmount * sysfs_mount;
 extern struct kmem_cache *sysfs_dir_cachep;