linux-kernel - Re: [patch] TTY: synclink, small cleanup in dtr

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Date:	Tue, 29 Jan 2013 10:55:16 -0500
From:	Valdis.Kletnieks@...edu
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Joe Perches <joe@...ches.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Paul Fulghum <paulkf@...rogate.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [patch] TTY: synclink, small cleanup in dtr_rts()

On Sun, 27 Jan 2013 23:19:47 +0300, Dan Carpenter said:

> Yeah.  I think it would be, but adding bitflags together instead of
> doing bitwise ORs is very common as well.

The fact it's common doesn't mean it's good programming practice,
or even correct.  Consider:

#define F_FOO 0x01
#define F_BAR 0x02
#define F_BAZ 0x04

unsigned int flags = F_FOO;
...
      flags |= F_BAR;

Now some time later, another code path does this:

      flags += F_FOO;

If it was another |, it would be a no harm no foul class of bug.
But how long is it going to take you to figure out who set F_BAZ?

I wonder if there's a way to write a coccinelle patch to find places
where we do arithmetic operations on bitmasks....

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