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

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Message-ID: <20210201121007.51c0ac59@gandalf.local.home>
Date:   Mon, 1 Feb 2021 12:10:07 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Ingo Molnar <mingo@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matt Mullins <mmullins@...x.us>, paulmck <paulmck@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v5] tracepoint: Do not fail unregistering a probe due to
 memory failure

On Mon, 1 Feb 2021 12:18:34 +1100
Alexey Kardashevskiy <aik@...abs.ru> 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