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

["Shi, Yang" <yang.shi@linaro.org>]

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
>