Re: [RFC V2 PATCH] kernfs: create raw version kernfs_path_len and kernfs_path

On 2/26/2016 3:01 PM, Greg KH wrote:

On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:

...

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 raw 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.

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 | 4 ++-- 4 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 996b774..70a9687 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(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(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..bbd88d3 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(struct cgroup *cgrp, char *buf,

• 			      size_t buflen)
  

+{

• return _kernfs_path(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..14c01cdc 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(struct kernfs_node *kn); char * __must_check kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen); +char * __must_check _kernfs_path(struct kernfs_node *kn, char *buf,

• 		size_t buflen);
  

I despise functions with just a _ in the front of them, you don't give anyone a clue as to what they are for, or why to use one vs. the other. Trust me, you will not remember it either in 1 month, let alone 5 years.

Please change the whole function name to be obvious as to what to use, or not use.

How about kernfs_*_unlocked() for raw version? And, keep lock version name intact?

Thanks, Yang

thanks,

greg k-h