Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

From: Miroslav Benes <mbenes@suse.cz>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Petr Mladek <pmladek@suse.com>,
	linux-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Jiri Kosina <jikos@kernel.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	live-patching@vger.kernel.org
Subject: Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback
Date: Fri, 30 Oct 2020 10:48:58 +0100 (CET)	[thread overview]
Message-ID: <alpine.LSU.2.21.2010301048080.22360@pobox.suse.cz> (raw)
In-Reply-To: <20201029142406.3c46855a@gandalf.local.home>

> > > > +	bit = ftrace_test_recursion_trylock();
> > > > +	if (bit < 0)
> > > > +		return;  
> > > 
> > > This means that the original function will be called in case of recursion. 
> > > That's probably fair, but I'm wondering if we should at least WARN about 
> > > it.  
> > 
> > Yeah, the early return might break the consistency model and
> > unexpected things might happen. We should be aware of it.
> > Please use:
> > 
> > 	if (WARN_ON_ONCE(bit < 0))
> > 		return;
> > 
> > WARN_ON_ONCE() might be part of the recursion. But it should happen
> > only once. IMHO, it is worth the risk.
> > 
> > Otherwise it looks good.
> 
> Perhaps we can add that as a separate patch, because this patch doesn't add
> any real functionality change. It only moves the recursion testing from the
> helper function (which ftrace wraps all callbacks that do not have the
> RECURSION flags set, including this one) down to your callback.
> 
> In keeping with one patch to do one thing principle, the added of
> WARN_ON_ONCE() should be a separate patch, as that will change the
> functionality.
> 
> If that WARN_ON_ONCE() breaks things, I'd like it to be bisected to another
> patch other than this one.

Works for me.

Miroslav