linux-kernel - Checking to see if a bit is _not

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Message-ID: <20141202021912.GA29096@thunk.org>
Date:	Mon, 1 Dec 2014 21:19:12 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Checking to see if a bit is _not_ set in a ftrace event filter

I was trying to do something like this:

filter="events/writeback/writeback_mark_inode_dirty/filter"
echo "(flags & 2048) && ((state & 2048) == 0)" > $filter

... but that doesn't work.

This works:

echo "flags & 2048" > $filter

But the problem is this:

echo "(state & 2048) == 0" > $filter

The simplest patch to add this would be add a new filter_ops so we
could do this:

echo "(state !& 2048)" > $filter

... but that's pretty ugly.  But adding more general expression
parsing in the ftrace event filter code would be non-trivial, and if
we start trying to make things like "!(state & 2048)" or "(state &
2048) == 0", then at some point some crazy person might request
supporting something like this: "(state ^ flags) == 2048".  :-)

So I guess the main question I want to ask is your opinion about
whether a patch that adds support for the operator "!&" is too ugly to
live?

Thanks,

					- Ted
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