Re: [PATCH 3/4] cgroup: fix locking in cgroup_cfts_commit()
On 2014/1/28 23:32, Tejun Heo wrote: > cgroup_cfts_commit() walks the cgroup hierarchy that the target > subsystem is attached to and tries to apply the file changes. Due to > the convolution with inode locking, it can't keep cgroup_mutex locked > while iterating. It currently holds only RCU read lock around the > actual iteration and then pins the found cgroup using dget(). > > Unfortunately, this is incorrect. Although the iteration does check > cgroup_is_dead() before invoking dget(), there's nothing which > prevents the dentry from going away inbetween. Note that this is > different from the usual css iterations where css_tryget() is used to > pin the css - css_tryget() tests whether the css can be pinned and > fails if not. > > The problem can be solved by simply holding cgroup_mutex instead of > RCU read lock around the iteration, which actually reduces LOC. > > Signed-off-by: Tejun Heo > Cc: sta...@vger.kernel.org Good catch! Acked-by: Li Zefan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[PATCH 3/4] cgroup: fix locking in cgroup_cfts_commit()
cgroup_cfts_commit() walks the cgroup hierarchy that the target
subsystem is attached to and tries to apply the file changes. Due to
the convolution with inode locking, it can't keep cgroup_mutex locked
while iterating. It currently holds only RCU read lock around the
actual iteration and then pins the found cgroup using dget().
Unfortunately, this is incorrect. Although the iteration does check
cgroup_is_dead() before invoking dget(), there's nothing which
prevents the dentry from going away inbetween. Note that this is
different from the usual css iterations where css_tryget() is used to
pin the css - css_tryget() tests whether the css can be pinned and
fails if not.
The problem can be solved by simply holding cgroup_mutex instead of
RCU read lock around the iteration, which actually reduces LOC.
Signed-off-by: Tejun Heo
Cc: sta...@vger.kernel.org
---
kernel/cgroup.c | 11 ++-
1 file changed, 2 insertions(+), 9 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2e9f12a..b0e030f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2763,10 +2763,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool
is_add)
*/
update_before = cgroup_serial_nr_next;
- mutex_unlock(_mutex);
-
/* add/rm files for all cgroups created before */
- rcu_read_lock();
css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
struct cgroup *cgrp = css->cgroup;
@@ -2775,23 +2772,19 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool
is_add)
inode = cgrp->dentry->d_inode;
dget(cgrp->dentry);
- rcu_read_unlock();
-
dput(prev);
prev = cgrp->dentry;
+ mutex_unlock(_mutex);
mutex_lock(>i_mutex);
mutex_lock(_mutex);
if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
ret = cgroup_addrm_files(cgrp, cfts, is_add);
- mutex_unlock(_mutex);
mutex_unlock(>i_mutex);
-
- rcu_read_lock();
if (ret)
break;
}
- rcu_read_unlock();
+ mutex_unlock(_mutex);
dput(prev);
deactivate_super(sb);
return ret;
--
1.8.5.3
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[PATCH 3/4] cgroup: fix locking in cgroup_cfts_commit()
cgroup_cfts_commit() walks the cgroup hierarchy that the target
subsystem is attached to and tries to apply the file changes. Due to
the convolution with inode locking, it can't keep cgroup_mutex locked
while iterating. It currently holds only RCU read lock around the
actual iteration and then pins the found cgroup using dget().
Unfortunately, this is incorrect. Although the iteration does check
cgroup_is_dead() before invoking dget(), there's nothing which
prevents the dentry from going away inbetween. Note that this is
different from the usual css iterations where css_tryget() is used to
pin the css - css_tryget() tests whether the css can be pinned and
fails if not.
The problem can be solved by simply holding cgroup_mutex instead of
RCU read lock around the iteration, which actually reduces LOC.
Signed-off-by: Tejun Heo t...@kernel.org
Cc: sta...@vger.kernel.org
---
kernel/cgroup.c | 11 ++-
1 file changed, 2 insertions(+), 9 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2e9f12a..b0e030f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2763,10 +2763,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool
is_add)
*/
update_before = cgroup_serial_nr_next;
- mutex_unlock(cgroup_mutex);
-
/* add/rm files for all cgroups created before */
- rcu_read_lock();
css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
struct cgroup *cgrp = css-cgroup;
@@ -2775,23 +2772,19 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool
is_add)
inode = cgrp-dentry-d_inode;
dget(cgrp-dentry);
- rcu_read_unlock();
-
dput(prev);
prev = cgrp-dentry;
+ mutex_unlock(cgroup_mutex);
mutex_lock(inode-i_mutex);
mutex_lock(cgroup_mutex);
if (cgrp-serial_nr update_before !cgroup_is_dead(cgrp))
ret = cgroup_addrm_files(cgrp, cfts, is_add);
- mutex_unlock(cgroup_mutex);
mutex_unlock(inode-i_mutex);
-
- rcu_read_lock();
if (ret)
break;
}
- rcu_read_unlock();
+ mutex_unlock(cgroup_mutex);
dput(prev);
deactivate_super(sb);
return ret;
--
1.8.5.3
--
To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 3/4] cgroup: fix locking in cgroup_cfts_commit()
On 2014/1/28 23:32, Tejun Heo wrote: cgroup_cfts_commit() walks the cgroup hierarchy that the target subsystem is attached to and tries to apply the file changes. Due to the convolution with inode locking, it can't keep cgroup_mutex locked while iterating. It currently holds only RCU read lock around the actual iteration and then pins the found cgroup using dget(). Unfortunately, this is incorrect. Although the iteration does check cgroup_is_dead() before invoking dget(), there's nothing which prevents the dentry from going away inbetween. Note that this is different from the usual css iterations where css_tryget() is used to pin the css - css_tryget() tests whether the css can be pinned and fails if not. The problem can be solved by simply holding cgroup_mutex instead of RCU read lock around the iteration, which actually reduces LOC. Signed-off-by: Tejun Heo t...@kernel.org Cc: sta...@vger.kernel.org Good catch! Acked-by: Li Zefan lize...@huawei.com -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
