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

Jiaxing Wang
2015-11-06T08:02:39Z

On Thu, Nov 05, 2015 at 10:54:32AM -0500, Steven Rostedt wrote:
> On Thu, 5 Nov 2015 13:23:01 +0800
> Jiaxing Wang <hello.wjx@gmail.com> wrote:
>
> > > > - /*
> > > > - * 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?
>
> Right.
>
> > 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.
>
> But this is still a failure. tracing_init_dentry() now only mounts
> tracefs on the debugfs/tracing directory. If it fails to do that when
> debugfs is available, then it should fail, as it would break backward
> compatibility with other tools.
>
> If debugfs is not configured in, then it should set tr->dir to
> whatever (ENOMEM is fine), and return NULL.
>
>
> > >
> > > > }
> > > >
> > > > + 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().
>
> What's wrong with that? This function is only to automount debugfs now.
>
> Also, I think the first check should be:
>
> if (WARN_ON(!tracefs_initialized()) ||
> (IS_ENABLED(CONFIG_DEBUGFS) &&
> WARN_ON(!debugfs_initialized()))
> return ERR_PTR(-ENODEV);
>
> Then we don't need the if (debugfs_initialized()) conditional.

I will send you a new patch according to your suggestion, and if that
is OK, I will send a seperate patch to Greg KH to add stub for
debugfs_create_automount().

Thanks.
>
> -- Steve
>
>
> > >
> > > -- Steve
> > >
> > > > return NULL;
> > > > }
> > > >
> > >
>
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