linux-kernel - Re: [GIT PULL] slab updates for 6.11

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Message-ID: <CAHk-=wjmu93njmUVqfkAbGKqHaOKFrTmgU2O=UkP3OOmpCjo4Q@mail.gmail.com>
Date: Wed, 18 Sep 2024 09:06:23 +0200
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: David Rientjes <rientjes@...gle.com>, Christoph Lameter <cl@...ux.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Hyeonggon Yoo <42.hyeyoo@...il.com>, Christian Brauner <brauner@...nel.org>, RCU <rcu@...r.kernel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [GIT PULL] slab updates for 6.11

On Mon, 16 Sept 2024 at 11:45, Vlastimil Babka <vbabka@...e.cz> wrote:
>
> There's a small conflict with the rcu tree:
> https://lore.kernel.org/lkml/20240812124748.3725011b@canb.auug.org.au/

Hmm. The conflict resolution is trivial, but the code itself looks buggy.

Look here, commit 2b55d6a42d14 ("rcu/kvfree: Add kvfree_rcu_barrier()
API") makes kvfree_rcu_queue_batch() do this:

        bool queued = false;
        ...
        for (i = 0; i < KFREE_N_BATCHES; i++) {
                ...
                        queued = queue_rcu_work(system_wq, &krwp->rcu_work);
        ...
        return queued;

and note how that return value is completely nonsensical. It doesn't
imply anything got queued. It's returning whether the *last* call to
queue_rcu_work() resulted in queued work.

There is no way the return value is meaningful that I can see, and
honestly, that means that the code in kvfree_rcu_barrier() looks
actively buggy, and at worst might be an endless loop

Now, maybe there's some reason why the code works fine, but it looks
really really wrong. Please fix.

The fix might be either a big comment about why it's ok, or making the
"queued" assignment be a '|=' instead, or perhaps breaking out of the
loop on the first successful queueing, or whatever.

But not this "randomly return _one_ value of many of the queuing success".

I've merged this, but I expect this to be fixed.

                  Linus