Messages in this thread Patch in this message | | | Date | Thu, 11 Dec 2008 10:23:43 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] CGroups: Fix a race between rmdir and remount |
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Paul Menage wrote: > Fix a race between rmdir and remount > > When a cgroup is removed, it's unlinked from its parent's children > list, but not actually freed until the last dentry on it is released > (at which point cgrp->root->number_of_cgroups is decremented). > > Currently rebind_subsystems checks for the top cgroup's child list > being empty in order to rebind subsystems into or out of a hierarchy - > this can result in the set of subsystems bound to a hierarchy being > different than the set of subsystems with state in the > removed-but-not-freed cgroup. > > The simplest fix for this is to forbid remounts that change the set of > subsystems on a hierarchy that has removed-but-not-freed cgroups. > > This bug can be reproduced via: >
Seems this bug is revealed by my patch:
cgroups-remove-some-redundant-null-checks.patch (http://marc.info/?l=linux-mm-commits&m=122730918427045&w=2)
@@ -611,10 +611,8 @@ static void cgroup_diput(struct dentry * ... - for_each_subsys(cgrp->root, ss) { - if (cgrp->subsys[ss->subsys_id]) - ss->destroy(ss, cgrp); - } + for_each_subsys(cgrp->root, ss) + ss->destroy(ss, cgrp);
But this patch is not guilty. :)
The original code leaked memory silently due to this race, if the remount removes some subsystems from the hierarchy.
> mkdir /mnt/cg > mount -t cgroup -o ns,freezer cgroup /mnt/cg > mkdir /mnt/cg/foo > sleep 1h < /mnt/cg/foo & > rmdir /mnt/cg/foo > mount -t cgroup -o remount,ns,devices,freezer cgroup /mnt/cg > kill $! > > Signed-off-by: Paul Menage <menage@google.com> >
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
> --- > kernel/cgroup.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c > =================================================================== > --- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c > +++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c > @@ -702,7 +702,7 @@ static int rebind_subsystems(struct cgro > * any child cgroups exist. This is theoretically supportable > * but involves complex error handling, so it's being left until > * later */ > - if (!list_empty(&cgrp->children)) > + if (root->number_of_cgroups > 1) > return -EBUSY; > > /* Process each subsystem */
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