Commit 231bed20 authored Mar 25, 2006 by Eric Dumazet Committed by Linus Torvalds Mar 25, 2006

[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>

parent 76c67de4

kernel/sys.c

+0 −6

Original line number	Diff line number	Diff line
		@@ -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;
		}