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SubjectRe: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task
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(ok crickets, keep the noise down please)

On Thu, 2011-04-28 at 11:38 +0200, Mike Galbraith wrote:

> The explosions are because the logic to snag rmdir() should anyone grab
> a reference will let us zip right through and free a cgroup while
> there's a destruction in flight. Adding a cgrp->count check before
> trying to cgroup_clear_css_refs() prevents the explosions, but that
> leaves RCU grace periods still embedded in userspace.
>
> So...
>
> I bent up put_css_set() a bit to try to destroy synchronously on final
> put if possible, so rmdir() will only be snagged if that fails. The
> thing seems to be working, so I'll show it. Readers (beware) may notice
> some gratuitous looking chicken scratches. Just ignore those, and move
> along smartly to the suggesting a much better way part, for surely one
> must exist.

Hi Self, (*howdy*)

You might try the below. No weird gyrations to accomplish the same
thing, and I see no slub debug gripes, no list debug gripes, nada.

Makes one wonder what these things do for a living.

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 25c7eb5..b8c64bf 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -826,13 +826,6 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
struct cgroup *cgrp = dentry->d_fsdata;
struct cgroup_subsys *ss;
BUG_ON(!(cgroup_is_removed(cgrp)));
- /* It's possible for external users to be holding css
- * reference counts on a cgroup; css_put() needs to
- * be able to access the cgroup after decrementing
- * the reference count in order to know if it needs to
- * queue the cgroup to be handled by the release
- * agent */
- synchronize_rcu();

mutex_lock(&cgroup_mutex);
/*
@@ -1822,7 +1815,6 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
ss->attach(ss, cgrp, oldcgrp, tsk, false);
}
set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
- synchronize_rcu();
put_css_set(cg);

/*



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