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@in... --- 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 77770ca..11c5acc 100644 --- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c +++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c @@ -2005,7 +2005,7 @@ 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); @@ -2415,7 +2415,7 @@ 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;

@@ -2568,7 +2568,7 @@ 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); rdt_last_cmd_clear(); if (!prdtgrp) { ret = -ENODEV; @@ -2643,7 +2643,7 @@ 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;

@@ -2654,7 +2654,7 @@ 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; }

@@ -2692,7 +2692,7 @@ 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; }

@@ -2735,7 +2735,7 @@ 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) { rdt_last_cmd_puts("kernfs subdir error\n"); goto out_del_list; @@ -2751,7 +2751,7 @@ 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; }