Re: [PATCH RESEND] tracing: Make tracing work when debugfs is not compiled or initialized.

[Jiaxing Wang]

On Wed, Nov 04, 2015 at 10:03:39AM -0500, Steven Rostedt wrote:
> On Wed,  4 Nov 2015 09:11:18 +0800
> Jiaxing Wang <hello.wjx@xxxxxxxxx> wrote:
> 
> > Currently tracing_init_dentry() returns -ENODEV when debugfs is not
> > initialized, which causes tracefs not populated with tracing files and
> > directories, so we will get an empty directory even after we manually
> > mount tracefs.
> > 
> > We can make tracing_init_dentry() return NULL as long as tracefs
> > is initialized and get a populated tracefs.
> > 
> > We also need to make global_trace.dir not NULL in order to pass the checks
> > in tracing_get_dentry() and add_tracer_options().
> > 
> > Also added stub debugfs_create_automount() for when debugfs is not
> > configured in.
> > 
> > Signed-off-by: Jiaxing Wang <hello.wjx@xxxxxxxxx>
> > ---
> >  include/linux/debugfs.h |  8 ++++++++
> >  kernel/trace/Kconfig    |  1 -
> >  kernel/trace/trace.c    | 29 +++++++++++++++++------------
> >  3 files changed, 25 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> > index 9beb636..b42ef88 100644
> > --- a/include/linux/debugfs.h
> > +++ b/include/linux/debugfs.h
> > @@ -160,6 +160,14 @@ static inline struct dentry *debugfs_create_symlink(const char *name,
> >  	return ERR_PTR(-ENODEV);
> >  }
> >  
> > +static inline struct dentry *debugfs_create_automount(const char *name,
> > +					struct dentry *parent,
> > +					struct vfsmount *(*f)(void *),
> > +					void *data)
> > +{
> > +	return ERR_PTR(-ENODEV);
> > +}
> 
> This part needs an Acked-by from Greg KH.
> 
> > +
> >  static inline void debugfs_remove(struct dentry *dentry)
> >  { }
> >  
> > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> > index 1153c43..59f6377f 100644
> > --- a/kernel/trace/Kconfig
> > +++ b/kernel/trace/Kconfig
> > @@ -95,7 +95,6 @@ config RING_BUFFER_ALLOW_SWAP
> >  
> >  config TRACING
> >  	bool
> > -	select DEBUG_FS
> >  	select RING_BUFFER
> >  	select STACKTRACE if STACKTRACE_SUPPORT
> >  	select TRACEPOINTS
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 6e79408..6dd064e 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -6706,6 +6706,8 @@ static struct vfsmount *trace_automount(void *ingore)
> >  	return mnt;
> >  }
> >  
> > +#define TRACE_TOP_DIR_ENTRY	((struct dentry *)1)
> > +
> >  /**
> >   * tracing_init_dentry - initialize top level trace array
> >   *
> > @@ -6716,27 +6718,30 @@ static struct vfsmount *trace_automount(void *ingore)
> >  struct dentry *tracing_init_dentry(void)
> >  {
> >  	struct trace_array *tr = &global_trace;
> > +	struct dentry *traced;
> >  
> >  	/* The top level trace array uses  NULL as parent */
> >  	if (tr->dir)
> >  		return NULL;
> >  
> > -	if (WARN_ON(!debugfs_initialized()))
> > +	if (WARN_ON(!tracefs_initialized()))
> >  		return ERR_PTR(-ENODEV);
> >  
> > -	/*
> > -	 * As there may still be users that expect the tracing
> > -	 * files to exist in debugfs/tracing, we must automount
> > -	 * the tracefs file system there, so older tools still
> > -	 * work with the newer kerenl.
> > -	 */
> > -	tr->dir = debugfs_create_automount("tracing", NULL,
> > -					   trace_automount, NULL);
> > -	if (!tr->dir) {
> > -		pr_warn_once("Could not create debugfs directory 'tracing'\n");
> > -		return ERR_PTR(-ENOMEM);
> > +	if (debugfs_initialized()) {
> > +		/*
> > +		 * As there may still be users that expect the tracing
> > +		 * files to exist in debugfs/tracing, we must automount
> > +		 * the tracefs file system there, so older tools still
> > +		 * work with the newer kerenl.
> > +		 */
> > +		traced = debugfs_create_automount("tracing", NULL,
> > +						   trace_automount, NULL);
> > +		if (!traced)
> > +			pr_warn_once("Could not create debugfs directory 'tracing'\n");
> 
> This should return a warning, and please keep the tr->dir instead of
> this new traced variable.
Do you mean return ERR_PTR(-ENOMEM); when debugfs_create_automount()
return NULL?
As long as tracefs is initialized, we can make tracing_init_dentry() return
NULL even if the debugfs automount point is not created(), and tracefs can
still be populated. If tracing_init_dentry() returns error in this case,
the caller of tracing_init_dentry() will not populate tracefs.
> 
> >  	}
> >  
> > +	tr->dir = TRACE_TOP_DIR_ENTRY;
> > +
> 
> Also, no need to add this, because if debugfs is not initialize, then
> tr->dir would be ERR_PTR(-ENODEV), which still works as tr->dir is not
> NULL.
If we accept debugfs_create_automount() return NULL, tr->dir can still
be NULL if we do tr->dir = debugfs_create_automount().
> 
> -- Steve
> 
> >  	return NULL;
> >  }
> >  
> 
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