On Tue, 22 May 2012 15:46:10 -0700
Andrew Morton [off-list ref] wrote:

quoted

+static inline bool memcg_proto_active(struct cg_proto *cg_proto)
+{
+	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVE);
+}
+
+static inline bool memcg_proto_activated(struct cg_proto *cg_proto)
+{
+	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVATED);
+}

Here, we're open-coding kinda-test_bit().  Why do that?  These flags are
modified with set_bit() and friends, so we should read them with the
matching test_bit()?

Also, these bool-returning functions will return values other than 0
and 1.  That probably works OK and I don't know what the C standards
and implementations do about this.  But it seems unclean and slightly
risky to have a "bool" value of 32!  Converting these functions to use
test_bit() fixes this - test_bit() returns only 0 or 1.

test_bit() is slightly more expensive than the above.  If this is
considered to be an issue then I guess we could continue to use this
approach.  But I do think a code comment is needed, explaining and
justifying the unusual decision to bypass the bitops API.  Also these
functions should tell the truth and return an "int" type.

Joe corrected (and informed) me:

: 6.3.1.2p1:
: 
: "When any scalar value is converted to _Bool, the result is 0
: if the value compares equal to 0; otherwise, the result is 1."

So the above functions will be given compiler-generated scalar-to-boolean
conversion.

test_bit() already does internal scalar-to-boolean conversion.  The
compiler doesn't know that, so if we convert the above functions to use
test_bit(), we'll end up performing scalar-to-boolean-to-boolean
conversion, which is dumb.

I assume that a way of fixing this is to change test_bit() to return
bool type.  That's a bit scary.

A less scary way would be to add a new

	bool test_bit_bool(int nr, const unsigned long *addr);

which internally calls test_bit() but somehow avoids the
compiler-generated conversion of the test_bit() return value into a
bool.  I haven't actually thought of a way of doing this ;)