commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update writeback tracepoints to report cgroup") made writeback tracepoints report cgroup writeback, but it may trigger the below bug on -rt kernel since kernfs_path and kernfs_path_len are called by tracepoints, which acquire sleeping lock.

BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930 in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3 INFO: lockdep is turned off. Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830

CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20 Hardware name: Freescale Layerscape 2085a RDB Board (DT) Workqueue: writeback wb_workfn (flush-7:0) Call trace: [<ffffffc00008d708>] dump_backtrace+0x0/0x200 [<ffffffc00008d92c>] show_stack+0x24/0x30 [<ffffffc0007b0f40>] dump_stack+0x88/0xa8 [<ffffffc000127d74>] ___might_sleep+0x2ec/0x300 [<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8 [<ffffffc0003e0548>] kernfs_path_len+0x30/0x90 [<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8 [<ffffffc000374f90>] wb_writeback+0x620/0x830 [<ffffffc000376224>] wb_workfn+0x61c/0x950 [<ffffffc000110adc>] process_one_work+0x3ac/0xb30 [<ffffffc0001112fc>] worker_thread+0x9c/0x7a8 [<ffffffc00011a9e8>] kthread+0x190/0x1b0 [<ffffffc000086ca0>] ret_from_fork+0x10/0x30

Since kernfs_* functions are heavily used by cgroup, so it sounds not reasonable to convert kernfs_rename_lock to raw lock.

Call synchronize_sched() when kernfs_node is updated since tracepoints are protected by rcu_read_lock_sched.

Create unlocked version kernfs_path, kernfs_path_len and cgroup_path, which don't acquire lock and are used by tracepoints only.

Signed-off-by: Yang Shi yang.shi@linaro.org --- It should be applicable to both mainline and -rt kernel. The change survives ftrace stress test in ltp.

v3: * Took Greg's comment to rename _kernfs_* and _cgroup_path to kernfs_*_unlocked and cgroup_path_unlocked.

v2: * Adopted Steven's suggestion to call synchronize_sched in kernfs_rename_ns.

fs/kernfs/dir.c | 30 +++++++++++++++++++++++++++--- include/linux/cgroup.h | 10 ++++++++++ include/linux/kernfs.h | 10 ++++++++++ include/trace/events/writeback.h | 5 +++-- 4 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 996b774..02b570b 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -44,7 +44,7 @@ static int kernfs_name_locked(struct kernfs_node *kn, char *buf, size_t buflen) return strlcpy(buf, kn->parent ? kn->name : "/", buflen); }

-static char * __must_check kernfs_path_locked(struct kernfs_node *kn, char *buf, +static char * __must_check __kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen) { char *p = buf + buflen; @@ -131,12 +131,33 @@ char *kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen) char *p;

spin_lock_irqsave(&kernfs_rename_lock, flags); - p = kernfs_path_locked(kn, buf, buflen); + p = __kernfs_path(kn, buf, buflen); spin_unlock_irqrestore(&kernfs_rename_lock, flags); return p; } EXPORT_SYMBOL_GPL(kernfs_path);

+/* Raw version. Used by tracepoints only without acquiring lock */ +size_t kernfs_path_len_unlocked(struct kernfs_node *kn) +{ + size_t len = 0; + + do { + len += strlen(kn->name) + 1; + kn = kn->parent; + } while (kn && kn->parent); + + return len; +} + +char *kernfs_path_unlocked(struct kernfs_node *kn, char *buf, size_t buflen) +{ + char *p; + + p = __kernfs_path(kn, buf, buflen); + return p; +} + /** * pr_cont_kernfs_name - pr_cont name of a kernfs_node * @kn: kernfs_node of interest @@ -168,7 +189,7 @@ void pr_cont_kernfs_path(struct kernfs_node *kn)

spin_lock_irqsave(&kernfs_rename_lock, flags);

- p = kernfs_path_locked(kn, kernfs_pr_cont_buf, + p = __kernfs_path(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf)); if (p) pr_cont("%s", p); @@ -1397,6 +1418,9 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent, kn->hash = kernfs_name_hash(kn->name, kn->ns); kernfs_link_sibling(kn);

+ /* Tracepoints are protected by rcu_read_lock_sched */ + synchronize_sched(); + kernfs_put(old_parent); kfree_const(old_name);

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h index 2162dca..52a19cd 100644 --- a/include/linux/cgroup.h +++ b/include/linux/cgroup.h @@ -538,6 +538,16 @@ static inline char * __must_check cgroup_path(struct cgroup *cgrp, char *buf, return kernfs_path(cgrp->kn, buf, buflen); }

+/* + * Without acquiring kernfs_rename_lock in _kernfs_path. + * Used by tracepoints only. + */ +static inline char * __must_check cgroup_path_unlocked(struct cgroup *cgrp, char *buf, + size_t buflen) +{ + return kernfs_path_unlocked(cgrp->kn, buf, buflen); +} + static inline void pr_cont_cgroup_name(struct cgroup *cgrp) { pr_cont_kernfs_name(cgrp->kn); diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h index af51df3..7c10530 100644 --- a/include/linux/kernfs.h +++ b/include/linux/kernfs.h @@ -267,8 +267,11 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)

int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen); size_t kernfs_path_len(struct kernfs_node *kn); +size_t kernfs_path_len_unlocked(struct kernfs_node *kn); char * __must_check kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen); +char * __must_check kernfs_path_unlocked(struct kernfs_node *kn, char *buf, + size_t buflen); void pr_cont_kernfs_name(struct kernfs_node *kn); void pr_cont_kernfs_path(struct kernfs_node *kn); struct kernfs_node *kernfs_get_parent(struct kernfs_node *kn); @@ -338,10 +341,17 @@ static inline int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen) static inline size_t kernfs_path_len(struct kernfs_node *kn) { return 0; }

+static inline size_t kernfs_path_len_unlocked(struct kernfs_node *kn) +{ return 0; } + static inline char * __must_check kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen) { return NULL; }

+static inline char * __must_check kernfs_path_unlocked(struct kernfs_node *kn, char *buf, + size_t buflen) +{ return NULL; } + static inline void pr_cont_kernfs_name(struct kernfs_node *kn) { } static inline void pr_cont_kernfs_path(struct kernfs_node *kn) { }

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h index fff846b..3620624 100644 --- a/include/trace/events/writeback.h +++ b/include/trace/events/writeback.h @@ -136,7 +136,7 @@ DEFINE_EVENT(writeback_dirty_inode_template, writeback_dirty_inode,

static inline size_t __trace_wb_cgroup_size(struct bdi_writeback *wb) { - return kernfs_path_len(wb->memcg_css->cgroup->kn) + 1; + return kernfs_path_len_unlocked(wb->memcg_css->cgroup->kn) + 1; }

static inline void __trace_wb_assign_cgroup(char *buf, struct bdi_writeback *wb) @@ -144,7 +144,8 @@ static inline void __trace_wb_assign_cgroup(char *buf, struct bdi_writeback *wb) struct cgroup *cgrp = wb->memcg_css->cgroup; char *path;

- path = cgroup_path(cgrp, buf, kernfs_path_len(cgrp->kn) + 1); + path = cgroup_path_unlocked(cgrp, buf, + kernfs_path_len_unlocked(cgrp->kn) + 1); WARN_ON_ONCE(path != buf); }