From: Marco Elver <elver@xxxxxxxxxx>
Date: Fri, 10 Jun 2022 18:32:36 +0200
On Fri, 10 Jun 2022 at 18:02, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
+/**
+ * generic_test_bit - Determine whether a bit is set
+ * @nr: bit number to test
+ * @addr: Address to start counting from
+ */
Shouldn't we add in this or in separate patch a big NOTE to explain that this
is actually atomic and must be kept as a such?
"atomic" isn't really the right word. The volatile access makes sure that the
compiler does the test at the point that the source code asked, and doesn't
move it before/after other operations.
It's listed in Documentation/atomic_bitops.txt.
Oh, so my memory was actually correct that I saw it in the docs
somewhere.
WDYT, should I mention this here in the code (block comment) as well
that it's atomic and must not lose `volatile` as Andy suggested or
it's sufficient to have it in the docs (+ it's not underscored)?
It is as "atomic" as READ_ONCE() or atomic_read() is. Though you are
right that the "atomicity" of reading one bit is almost a given,
because we can't really read half a bit.
The main thing is that the compiler keeps it "atomic" and e.g. doesn't
fuse the load with another or elide it completely, and then transforms
the code in concurrency-unfriendly ways.
Like READ_ONCE() and friends, test_bit(), unlike non-atomic bitops,
may also be used to dependency-order some subsequent marked (viz.
atomic) operations.
But there is no such thing as an atomic test_bit() operation:
if (test_bit(5, addr)) {
/* some other CPU nukes bit 5 */
/* I know it was set when I looked, but now, could be anything */
The operation itself is atomic, because reading half a bit is
impossible. Whether or not that bit is modified concurrently is a
different problem.
Thanks,
-- Marco
Thanks,
Olek
