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>

2 files changed, 50 insertions, 37 deletions

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index f5f0b936f181..40596a0eee52 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -10,6 +10,7 @@
 #include <linux/kobject.h>
 #include <linux/namei.h>
 #include <linux/idr.h>
+#include <linux/completion.h>
 #include <asm/semaphore.h>
 #include "sysfs.h"
 
@@ -32,11 +33,24 @@ static DEFINE_IDA(sysfs_ino_ida);
  */
 struct sysfs_dirent *sysfs_get_active(struct sysfs_dirent *sd)
 {
-        if (sd) {
-                if (unlikely(!down_read_trylock(&sd->s_active)))
-                        sd = NULL;
+        if (unlikely(!sd))
+                return NULL;
+
+        while (1) {
+                int v, t;
+
+                v = atomic_read(&sd->s_active);
+                if (unlikely(v < 0))
+                        return NULL;
+
+                t = atomic_cmpxchg(&sd->s_active, v, v + 1);
+                if (likely(t == v))
+                        return sd;
+                if (t < 0)
+                        return NULL;
+
+                cpu_relax();
         }
-        return sd;
 }
 
 /**
@@ -48,8 +62,21 @@ struct sysfs_dirent *sysfs_get_active(struct sysfs_dirent *sd)
  */
 void sysfs_put_active(struct sysfs_dirent *sd)
 {
-        if (sd)
-                up_read(&sd->s_active);
+        struct completion *cmpl;
+        int v;
+
+        if (unlikely(!sd))
+                return;
+
+        v = atomic_dec_return(&sd->s_active);
+        if (likely(v != SD_DEACTIVATED_BIAS))
+                return;
+
+        /* atomic_dec_return() is a mb(), we'll always see the updated
+         * sd->s_sibling.next.
+         */
+        cmpl = (void *)sd->s_sibling.next;
+        complete(cmpl);
 }
 
 /**
@@ -95,17 +122,25 @@ void sysfs_put_active_two(struct sysfs_dirent *sd)
  *      sysfs_deactivate - deactivate sysfs_dirent
  *      @sd: sysfs_dirent to deactivate
  *
- *      Deny new active references and drain existing ones.  s_active
- *      will be unlocked when the sysfs_dirent is released.
+ *      Deny new active references and drain existing ones.
  */
 void sysfs_deactivate(struct sysfs_dirent *sd)
 {
-        down_write_nested(&sd->s_active, SYSFS_S_ACTIVE_DEACTIVATE);
+        DECLARE_COMPLETION_ONSTACK(wait);
+        int v;
+
+        BUG_ON(!list_empty(&sd->s_sibling));
+        sd->s_sibling.next = (void *)&wait;
 
-        /* s_active will be unlocked by the thread doing the final put
-         * on @sd.  Lie to lockdep.
+        /* atomic_add_return() is a mb(), put_active() will always see
+         * the updated sd->s_sibling.next.
          */
-        rwsem_release(&sd->s_active.dep_map, 1, _RET_IP_);
+        v = atomic_add_return(SD_DEACTIVATED_BIAS, &sd->s_active);
+
+        if (v != SD_DEACTIVATED_BIAS)
+                wait_for_completion(&wait);
+
+        INIT_LIST_HEAD(&sd->s_sibling);
 }
 
 static int sysfs_alloc_ino(ino_t *pino)
@@ -141,19 +176,6 @@ void release_sysfs_dirent(struct sysfs_dirent * sd)
  repeat:
         parent_sd = sd->s_parent;
 
-        /* If @sd is being released after deletion, s_active is write
-         * locked.  If @sd is cursor for directory walk or being
-         * released prematurely, s_active has no reader or writer.
-         *
-         * sysfs_deactivate() lies to lockdep that s_active is
-         * unlocked immediately.  Lie one more time to cover the
-         * previous lie.
-         */
-        if (!down_write_trylock(&sd->s_active))
-                rwsem_acquire(&sd->s_active.dep_map,
-                              SYSFS_S_ACTIVE_DEACTIVATE, 0, _RET_IP_);
-        up_write(&sd->s_active);
-
         if (sd->s_type & SYSFS_KOBJ_LINK)
                 sysfs_put(sd->s_elem.symlink.target_sd);
         if (sd->s_type & SYSFS_COPY_NAME)
@@ -213,8 +235,8 @@ struct sysfs_dirent *sysfs_new_dirent(const char *name, umode_t mode, int type)
                 goto err_out;
 
         atomic_set(&sd->s_count, 1);
+        atomic_set(&sd->s_active, 0);
         atomic_set(&sd->s_event, 1);
-        init_rwsem(&sd->s_active);
         INIT_LIST_HEAD(&sd->s_children);
         INIT_LIST_HEAD(&sd->s_sibling);
 
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;