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

Jiaxing Wang
2015-11-05T05:23:09Z

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@gmail.com> 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@gmail.com>
> > ---
> > 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;
> > }
> >
>