On Wed, Jul 15, 2020 at 09:13:42AM -0700, Eric Biggers wrote:

On Wed, Jul 15, 2020 at 06:01:44PM +1000, Dave Chinner wrote:

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 /* direct-io.c: */
-int sb_init_dio_done_wq(struct super_block *sb);
+int __sb_init_dio_done_wq(struct super_block *sb);
+static inline int sb_init_dio_done_wq(struct super_block *sb)
+{
+	/* pairs with cmpxchg() in __sb_init_dio_done_wq() */
+	if (likely(READ_ONCE(sb->s_dio_done_wq)))
+		return 0;
+	return __sb_init_dio_done_wq(sb);
+}

Ummm, why don't you just add this check in sb_init_dio_done_wq(). I
don't see any need for adding another level of function call
abstraction in the source code?

This keeps the fast path doing no function calls and one fewer branch, as it was
before.  People care a lot about minimizing direct I/O overhead, so it seems
desirable to keep this simple optimization.  Would you rather it be removed?

No.

What I'm trying to say is that I'd prefer fast path checks don't get
hidden away in a static inline function wrappers that require the
reader to go look up code in a different file to understand that
code in yet another different file is conditionally executed.

Going from obvious, easy to read fast path code to spreading the
fast path logic over functions in 3 different files is not an
improvement in the code - it is how we turn good code into an
unmaintainable mess...

The alternative would be to duplicate the READ_ONCE() at all 3 call sites --
including the explanatory comment.  That seems strictly worse.

And the code before was broken, so I disagree it was "obvious" or "good".

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Also, you need to explain the reason for the READ_ONCE() existing
rather than just saying "it pairs with <some other operation>".
Knowing what operation it pairs with doesn't explain why the pairing
is necessary in the first place, and that leads to nobody reading
the code being able to understand what this is protecting against.

How about this?

	/*
	 * Nothing to do if ->s_dio_done_wq is already set.  But since another
	 * process may set it concurrently, we need to use READ_ONCE() rather
	 * than a plain read to avoid a data race (undefined behavior) and to
	 * ensure we observe the pointed-to struct to be fully initialized.
	 */
	if (likely(READ_ONCE(sb->s_dio_done_wq)))
		return 0;

You still need to document what it pairs with, as "data race" doesn't
describe the actual dependency we are synchronising against is.

AFAICT from your description, the data race is not on
sb->s_dio_done_wq itself, but on seeing the contents of the
structure being pointed to incorrectly. i.e. we need to ensure that
writes done before the cmpxchg are ordered correctly against
reads done after the pointer can be seen here.

No, the data race is on ->s_dio_done_wq itself.  How about this:

        /*
         * Nothing to do if ->s_dio_done_wq is already set.  The READ_ONCE()
         * here pairs with the cmpxchg() in __sb_init_dio_done_wq().  Since the
         * cmpxchg() may set ->s_dio_done_wq concurrently, a plain load would be
         * a data race (undefined behavior), so READ_ONCE() is needed.
         * READ_ONCE() also includes any needed read data dependency barrier to
         * ensure that the pointed-to struct is seen to be fully initialized.
         */

FWIW, long-term we really need to get developers to understand these sorts of
issues, so that the code is written correctly in the first place and we don't
need to annotate common patterns like one-time-init with a long essay and have a
long discussion.  Recently KCSAN was merged upstream
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/dev-tools/kcsan.rst)
and the memory model documentation was improved
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt?h=v5.8-rc5#n1922),
so hopefully that will raise awareness...

I tried to understand that, but TBH this whole topic area seems very
complex and difficult to understand.  I more or less understand what
READ_ONCE and WRITE_ONCE do wrt restricting compiler optimizations, but
I wouldn't say that I understand all that machinery.

Granted, my winning strategy so far is to write a simple version with
big dumb locks and let the rest of you argue over slick optimizations.
:P

<shrug> If using READ_ONCE and cmpxchg for pointer initialization (or I
guess smp_store_release and smp_load_acquire?) are a commonly used
paradigm, then maybe that should get its own section in
tools/memory-model/Documentation/recipes.txt and then any code that uses
it can point readers at that?

--D

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If so, can't we just treat this as a normal
store-release/load-acquire ordering pattern and hence use more
relaxed memory barriers instead of have to patch up what we have now
to specifically make ancient platforms that nobody actually uses
with weird and unusual memory models work correctly?

READ_ONCE() is already as relaxed as it can get, as it includes a read data
dependency barrier only (which is no-op on everything other than Alpha).

If anything it should be upgraded to smp_load_acquire(), which handles control
dependencies too.  I didn't see anything obvious in the workqueue code that
would need that (i.e. accesses to some global structure that isn't transitively
reachable via the workqueue_struct itself).  But we could use it to be safe if
we're okay with any performance implications of the additional memory barrier it
would add.

- Eric