[PATCH] No need to protect current->group_info in sys_getgroups(), in_group_p() and in_egroup_p()

While doing some benchmarks of an Apache/PHP SMP server, I noticed high
oprofile numbers in in_group_p() and _atomic_dec_and_lock().

rank  percent
  1     4.8911 % __link_path_walk
  2     4.8503 % __d_lookup
*3     4.2911 % _atomic_dec_and_lock
  4     3.9307 % __copy_to_user_ll
  5     4.9004 % sysenter_past_esp
*6     3.3248 % in_group_p

It appears that in_group_p() does an uncessary

get_group_info(current->group_info); /* atomic_inc() */
  ... /* access current->group_info */
put_group_info(current->group_info); /* _atomic_dec_and_lock */

It is not necessary to do this, because the current task holds a reference
on its own group_info, and this reference cannot change during the lookup.

This patch deletes the get_group_info()/put_group_info() pair from
sys_getgroups(), in_group_p() and in_egroup_p() functions.
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Cc: Tim Hockin <thockin@hockin.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sys.c

@@ -1421,7 +1421,6 @@ asmlinkage long sys_getgroups(int gidsetsize, gid_t __user *grouplist)
 		return -EINVAL;
 
 	/* no need to grab task_lock here; it cannot change */
-	get_group_info(current->group_info);
 	i = current->group_info->ngroups;
 	if (gidsetsize) {
 		if (i > gidsetsize) {
@@ -1434,7 +1433,6 @@ asmlinkage long sys_getgroups(int gidsetsize, gid_t __user *grouplist)
 		}
 	}
 out:
-	put_group_info(current->group_info);
 	return i;
 }
 
@@ -1475,9 +1473,7 @@ int in_group_p(gid_t grp)
 {
 	int retval = 1;
 	if (grp != current->fsgid) {
-		get_group_info(current->group_info);
 		retval = groups_search(current->group_info, grp);
-		put_group_info(current->group_info);
 	}
 	return retval;
 }
@@ -1488,9 +1484,7 @@ int in_egroup_p(gid_t grp)
 {
 	int retval = 1;
 	if (grp != current->egid) {
-		get_group_info(current->group_info);
 		retval = groups_search(current->group_info, grp);
-		put_group_info(current->group_info);
 	}
 	return retval;
 }