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Subject[PATCH 4.14 36/89] x86/resctrl: Fix a deadlock due to inaccurate reference
Date
From: Xiaochen Shen <xiaochen.shen@intel.com>

commit 334b0f4e9b1b4a1d475f803419d202f6c5e4d18e upstream.

There is a race condition which results in a deadlock when rmdir and
mkdir execute concurrently:

$ ls /sys/fs/resctrl/c1/mon_groups/m1/
cpus cpus_list mon_data tasks

Thread 1: rmdir /sys/fs/resctrl/c1
Thread 2: mkdir /sys/fs/resctrl/c1/mon_groups/m1

3 locks held by mkdir/48649:
#0: (sb_writers#17){.+.+}, at: [<ffffffffb4ca2aa0>] mnt_want_write+0x20/0x50
#1: (&type->i_mutex_dir_key#8/1){+.+.}, at: [<ffffffffb4c8c13b>] filename_create+0x7b/0x170
#2: (rdtgroup_mutex){+.+.}, at: [<ffffffffb4a4389d>] rdtgroup_kn_lock_live+0x3d/0x70

4 locks held by rmdir/48652:
#0: (sb_writers#17){.+.+}, at: [<ffffffffb4ca2aa0>] mnt_want_write+0x20/0x50
#1: (&type->i_mutex_dir_key#8/1){+.+.}, at: [<ffffffffb4c8c3cf>] do_rmdir+0x13f/0x1e0
#2: (&type->i_mutex_dir_key#8){++++}, at: [<ffffffffb4c86d5d>] vfs_rmdir+0x4d/0x120
#3: (rdtgroup_mutex){+.+.}, at: [<ffffffffb4a4389d>] rdtgroup_kn_lock_live+0x3d/0x70

Thread 1 is deleting control group "c1". Holding rdtgroup_mutex,
kernfs_remove() removes all kernfs nodes under directory "c1"
recursively, then waits for sub kernfs node "mon_groups" to drop active
reference.

Thread 2 is trying to create a subdirectory "m1" in the "mon_groups"
directory. The wrapper kernfs_iop_mkdir() takes an active reference to
the "mon_groups" directory but the code drops the active reference to
the parent directory "c1" instead.

As a result, Thread 1 is blocked on waiting for active reference to drop
and never release rdtgroup_mutex, while Thread 2 is also blocked on
trying to get rdtgroup_mutex.

Thread 1 (rdtgroup_rmdir) Thread 2 (rdtgroup_mkdir)
(rmdir /sys/fs/resctrl/c1) (mkdir /sys/fs/resctrl/c1/mon_groups/m1)
------------------------- -------------------------
kernfs_iop_mkdir
/*
* kn: "m1", parent_kn: "mon_groups",
* prgrp_kn: parent_kn->parent: "c1",
*
* "mon_groups", parent_kn->active++: 1
*/
kernfs_get_active(parent_kn)
kernfs_iop_rmdir
/* "c1", kn->active++ */
kernfs_get_active(kn)

rdtgroup_kn_lock_live
atomic_inc(&rdtgrp->waitcount)
/* "c1", kn->active-- */
kernfs_break_active_protection(kn)
mutex_lock

rdtgroup_rmdir_ctrl
free_all_child_rdtgrp
sentry->flags = RDT_DELETED

rdtgroup_ctrl_remove
rdtgrp->flags = RDT_DELETED
kernfs_get(kn)
kernfs_remove(rdtgrp->kn)
__kernfs_remove
/* "mon_groups", sub_kn */
atomic_add(KN_DEACTIVATED_BIAS, &sub_kn->active)
kernfs_drain(sub_kn)
/*
* sub_kn->active == KN_DEACTIVATED_BIAS + 1,
* waiting on sub_kn->active to drop, but it
* never drops in Thread 2 which is blocked
* on getting rdtgroup_mutex.
*/
Thread 1 hangs here ---->
wait_event(sub_kn->active == KN_DEACTIVATED_BIAS)
...
rdtgroup_mkdir
rdtgroup_mkdir_mon(parent_kn, prgrp_kn)
mkdir_rdt_prepare(parent_kn, prgrp_kn)
rdtgroup_kn_lock_live(prgrp_kn)
atomic_inc(&rdtgrp->waitcount)
/*
* "c1", prgrp_kn->active--
*
* The active reference on "c1" is
* dropped, but not matching the
* actual active reference taken
* on "mon_groups", thus causing
* Thread 1 to wait forever while
* holding rdtgroup_mutex.
*/
kernfs_break_active_protection(
prgrp_kn)
/*
* Trying to get rdtgroup_mutex
* which is held by Thread 1.
*/
Thread 2 hangs here ----> mutex_lock
...

The problem is that the creation of a subdirectory in the "mon_groups"
directory incorrectly releases the active protection of its parent
directory instead of itself before it starts waiting for rdtgroup_mutex.
This is triggered by the rdtgroup_mkdir() flow calling
rdtgroup_kn_lock_live()/rdtgroup_kn_unlock() with kernfs node of the
parent control group ("c1") as argument. It should be called with kernfs
node "mon_groups" instead. What is currently missing is that the
kn->priv of "mon_groups" is NULL instead of pointing to the rdtgrp.

Fix it by pointing kn->priv to rdtgrp when "mon_groups" is created. Then
it could be passed to rdtgroup_kn_lock_live()/rdtgroup_kn_unlock()
instead. And then it operates on the same rdtgroup structure but handles
the active reference of kernfs node "mon_groups" to prevent deadlock.
The same changes are also made to the "mon_data" directories.

This results in some unused function parameters that will be cleaned up
in follow-up patch as the focus here is on the fix only in support of
backporting efforts.

Backporting notes:

Since upstream commit fa7d949337cc ("x86/resctrl: Rename and move rdt
files to a separate directory"), the file
arch/x86/kernel/cpu/intel_rdt_rdtgroup.c has been renamed and moved to
arch/x86/kernel/cpu/resctrl/rdtgroup.c.
Apply the change against file arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
for older stable trees.

Fixes: c7d9aac61311 ("x86/intel_rdt/cqm: Add mkdir support for RDT monitoring")
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1578500886-21771-4-git-send-email-xiaochen.shen@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kernel/cpu/intel_rdt_rdtgroup.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
index 01574966d91fd..0ec30b2384c05 100644
--- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
+++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
@@ -1107,7 +1107,7 @@ static struct dentry *rdt_mount(struct file_system_type *fs_type,

if (rdt_mon_capable) {
ret = mongroup_create_dir(rdtgroup_default.kn,
- NULL, "mon_groups",
+ &rdtgroup_default, "mon_groups",
&kn_mongrp);
if (ret) {
dentry = ERR_PTR(ret);
@@ -1499,7 +1499,7 @@ static int mkdir_mondata_all(struct kernfs_node *parent_kn,
/*
* Create the mon_data directory first.
*/
- ret = mongroup_create_dir(parent_kn, NULL, "mon_data", &kn);
+ ret = mongroup_create_dir(parent_kn, prgrp, "mon_data", &kn);
if (ret)
return ret;

@@ -1533,7 +1533,7 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
uint files = 0;
int ret;

- prdtgrp = rdtgroup_kn_lock_live(prgrp_kn);
+ prdtgrp = rdtgroup_kn_lock_live(parent_kn);
if (!prdtgrp) {
ret = -ENODEV;
goto out_unlock;
@@ -1589,7 +1589,7 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
kernfs_activate(kn);

/*
- * The caller unlocks the prgrp_kn upon success.
+ * The caller unlocks the parent_kn upon success.
*/
return 0;

@@ -1600,7 +1600,7 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
out_free_rgrp:
kfree(rdtgrp);
out_unlock:
- rdtgroup_kn_unlock(prgrp_kn);
+ rdtgroup_kn_unlock(parent_kn);
return ret;
}

@@ -1638,7 +1638,7 @@ static int rdtgroup_mkdir_mon(struct kernfs_node *parent_kn,
*/
list_add_tail(&rdtgrp->mon.crdtgrp_list, &prgrp->mon.crdtgrp_list);

- rdtgroup_kn_unlock(prgrp_kn);
+ rdtgroup_kn_unlock(parent_kn);
return ret;
}

@@ -1675,7 +1675,7 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn,
* Create an empty mon_groups directory to hold the subset
* of tasks and cpus to monitor.
*/
- ret = mongroup_create_dir(kn, NULL, "mon_groups", NULL);
+ ret = mongroup_create_dir(kn, rdtgrp, "mon_groups", NULL);
if (ret)
goto out_id_free;
}
@@ -1688,7 +1688,7 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn,
out_common_fail:
mkdir_rdt_prepare_clean(rdtgrp);
out_unlock:
- rdtgroup_kn_unlock(prgrp_kn);
+ rdtgroup_kn_unlock(parent_kn);
return ret;
}

--
2.20.1


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