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

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

[Yang Shi]

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);
 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(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(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..91ceced 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(wb->memcg_css->cgroup->kn) + 1;
 }
 
 static inline void __trace_wb_assign_cgroup(char *buf, struct bdi_writeback *wb)
@@ -144,7 +144,7 @@ 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(cgrp, buf, _kernfs_path_len(cgrp->kn) + 1);
 	WARN_ON_ONCE(path != buf);
 }
 
-- 
2.0.2

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

[Greg KH]

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.

thanks,

greg k-h

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

[Shi, Yang]

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
>

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

[Christoph Hellwig]

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.

Sounds like the tracepoints simplify shouldn't try to do this path
generation at all..

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

[Tejun Heo]

Hello,

On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:
> Call synchronize_sched() when kernfs_node is updated since tracepoints are
> protected by rcu_read_lock_sched.

Adding synchronize_sched() to operations which can be triggered from
userland usually turns out to be problematic.  If this can't be solved
any other way, I think the right thing to do is replacing the path
with cgroup id as Christoph suggested.

Thanks.

-- 
tejun

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

[Thomas Gleixner]

On Sat, 27 Feb 2016, Tejun Heo wrote:
> On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:
> > Call synchronize_sched() when kernfs_node is updated since tracepoints are
> > protected by rcu_read_lock_sched.
> 
> Adding synchronize_sched() to operations which can be triggered from
> userland usually turns out to be problematic.  If this can't be solved
> any other way, I think the right thing to do is replacing the path
> with cgroup id as Christoph suggested.

Agreed. The path is not that important, right?

Thanks,

	tglx

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

[Tejun Heo]

On Sat, Feb 27, 2016 at 12:37:23PM +0100, Thomas Gleixner wrote:
> On Sat, 27 Feb 2016, Tejun Heo wrote:
> > On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:
> > > Call synchronize_sched() when kernfs_node is updated since tracepoints are
> > > protected by rcu_read_lock_sched.
> > 
> > Adding synchronize_sched() to operations which can be triggered from
> > userland usually turns out to be problematic.  If this can't be solved
> > any other way, I think the right thing to do is replacing the path
> > with cgroup id as Christoph suggested.
> 
> Agreed. The path is not that important, right?

It can be, but we can print out the ino and userland can match that up
with path if necessary.

Thanks.

-- 
tejun

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

[Thomas Gleixner]

On Sat, 27 Feb 2016, Tejun Heo wrote:

> On Sat, Feb 27, 2016 at 12:37:23PM +0100, Thomas Gleixner wrote:
> > On Sat, 27 Feb 2016, Tejun Heo wrote:
> > > On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:
> > > > Call synchronize_sched() when kernfs_node is updated since tracepoints are
> > > > protected by rcu_read_lock_sched.
> > > 
> > > Adding synchronize_sched() to operations which can be triggered from
> > > userland usually turns out to be problematic.  If this can't be solved
> > > any other way, I think the right thing to do is replacing the path
> > > with cgroup id as Christoph suggested.
> > 
> > Agreed. The path is not that important, right?
> 
> It can be, but we can print out the ino and userland can match that up
> with path if necessary.

Wouldn't be cgroup id the better choice?

Thanks,

	tglx

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

[Tejun Heo]

Hello,

On Sat, Feb 27, 2016 at 6:45 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> It can be, but we can print out the ino and userland can match that up
>> with path if necessary.
>
> Wouldn't be cgroup id the better choice?

AFAIK we aren't exposing cgroup id to userland anywhere right now.
Eventually, I think the right thing to do is using the same number for
both.

Thanks.

-- 
tejun

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

[Shi, Yang]

On 2/27/2016 3:51 AM, Tejun Heo wrote:
> Hello,
>
> On Sat, Feb 27, 2016 at 6:45 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>>> It can be, but we can print out the ino and userland can match that up
>>> with path if necessary.
>>
>> Wouldn't be cgroup id the better choice?
>
> AFAIK we aren't exposing cgroup id to userland anywhere right now.
> Eventually, I think the right thing to do is using the same number for
> both.

Thanks for all the comments.

Since the current tracepoints print the path length too by 
__trace_wb_cgroup_size and __trace_wbc_cgroup_size, but we can't get the 
path length if we switch to group ino. So, I'm supposed I have to drop 
all the *_size stuff.

And, when CONFIG_CGROUP_WRITEBACK is not enabled, 
__trace_wb_assign_cgroup and __trace_wbc_assign_cgroup return 
"strcpy(buf, "/")", so to get aligned with this, I need print out the 
ino of "/", right? But, the ROOT_INO may vary from different filesystems.

All of them will be addressed in V4, but it may experience some delay 
since I have to travel this week.

Regards,
Yang

>
> Thanks.
>