Re: [PATCH v5] tracepoint: Do not fail unregistering a probe due to memory failure

[Steven Rostedt]

On Mon, 1 Feb 2021 12:18:34 +1100
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:

> > Just curious, does the following patch fix it for v5?  
> 
> 
> Yes it does!

Thanks for verifying.

> 
> 
> > 
> > -- Steve
> > 
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 7261fa0f5e3c..cf3a6d104fdb 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -306,6 +306,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> >   			tp->unregfunc();
> >   
> >   		static_key_disable(&tp->key);
> > +		tracepoint_synchronize_unregister();
> >   		rcu_assign_pointer(tp->funcs, tp_funcs);
> >   	} else {
> >   		rcu_assign_pointer(tp->funcs, tp_funcs);
> >   

OK, since it would be expensive to do a synchronization on every removal
like that, but the tp->funcs should not be reset.

It appears that your check is still required, since the iterator has been
added.

The quick fix is the check you gave.

But I think we could optimize this (not having to dereference the array
twice, and do the check twice) by making the iterator part of the tp_funcs
array, and having the rest of the array as its argument. But that can be a
separate update.

The check you added should be a patch and marked for stable. Care to send
it, and mark it for stable as well as:

Fixes: d25e37d89dd2f ("tracepoint: Optimize using static_call()")

Thanks!

-- Steve