linux-kernel - Re: [PATCH 001/141] afs: Fix fall-through warnings for Clang

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Date:   Mon, 23 Nov 2020 08:51:14 -0800
From:   Joe Perches <joe@...ches.com>
To:     David Howells <dhowells@...hat.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-afs@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 001/141] afs: Fix fall-through warnings for Clang

On Mon, 2020-11-23 at 16:10 +0000, David Howells wrote:
> Joe Perches <joe@...ches.com> wrote:
> 
> > >  		call->unmarshall++;
> > > +
> > > +		fallthrough;
> > 
> > My preference would be to change these to break and not fallthrough;
> > 
> > >  	case 5:
> > >  		break;
> > >  	}
> 
> My preference would be to fall through.  The case number is the state machine
> state, as indexed by call->unmarshall.

Then ideally the state machine states should be enums and not numbers
and the compiler should use a default block for unhandled states right?

Is code like call->marshall++ a common style for kernel state machines?
Perhaps not.

Does it work?
Sure.

Is it obvious what the transitions are?
No.

> All the other cases in the switch fall through.
> 
> I would also generally prefer that the fallthrough statement occur before the
> blank line, not after it, since it belongs with the previous clause, and not
> between a comment about a case statement and its associated case statement,
> i.e.:
> 
> 		afs_extract_to_tmp(call);
> 		call->unmarshall++;
> 
> 		/* extract the callback array and its count in two steps */
> 		fallthrough;
> 	case 3:
> 
> would be better written as:
> 
> 		afs_extract_to_tmp(call);
> 		call->unmarshall++;
> 		fallthrough;
> 
> 		/* extract the callback array and its count in two steps */
> 	case 3:

I agree completely.