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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme
Date: Fri, 10 Dec 2021 09:27:48 -0800	[thread overview]
Message-ID: <CAHk-=whStLWvUzmz3SmQxy1PPyNDjf1O-7z1mq5=WZ-+EABa7w@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiDXWy8ekFDxVzCbudZv_3CqiWa9w+xO8mxJkk8SNmJCg@mail.gmail.com>

On Fri, Dec 10, 2021 at 8:53 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> That said - it may not matter - I'm not sure a plain "dec" is even a
> valid operation on a ref in the first place. How could you ever
> validly decrement a ref without checking for it being the last entry?

I should have checked the users - it seems to be a pattern at least in
networking where people have extra references and do

        refcount_dec(&skb->users);
        dev_kfree_skb_any(skb);

because there's no way to tell dev_kfree_skb*() to decrement more than once.

So I guess it's all good, but yes, I still think you can just do "lock
dec .. js" for this operation.

             Linus

  reply	other threads:[~2021-12-10 17:28 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-10 16:16 [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 1/9] atomic: Prepare scripts for macro ops Peter Zijlstra
2021-12-10 17:27   ` Mark Rutland
2021-12-10 17:43   ` Marco Elver
2021-12-10 16:16 ` [PATCH v2 2/9] atomic: Add xchg.tbl Peter Zijlstra
2021-12-13  9:50   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 3/9] atomic: Introduce atomic_{inc,dec,dec_and_test}_overflow() Peter Zijlstra
2021-12-13 10:06   ` Mark Rutland
2021-12-13 10:57     ` Peter Zijlstra
2021-12-13 10:59     ` Peter Zijlstra
2021-12-13 11:09       ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 4/9] refcount: Use atomic_*_overflow() Peter Zijlstra
2021-12-13 10:35   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 5/9] atomic,x86: Implement atomic_dec_and_test_overflow() Peter Zijlstra
2021-12-13 11:04   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 6/9] refcount: Fix refcount_dec_not_one() Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 7/9] refcount: Prepare for atomic_*_overflow() offsets Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme Peter Zijlstra
2021-12-10 16:53   ` Linus Torvalds
2021-12-10 17:27     ` Linus Torvalds [this message]
2021-12-17  3:38     ` Herbert Xu
2021-12-13 16:43   ` Peter Zijlstra
2021-12-13 17:29     ` Marco Elver
2021-12-13 18:11     ` Linus Torvalds
2021-12-13 18:18       ` Marco Elver
2021-12-13 18:24         ` Linus Torvalds
2021-12-13 19:35           ` Marco Elver
2021-12-13 18:21       ` Linus Torvalds
2021-12-10 16:16 ` [PATCH v2 9/9] refcount: Optimize __refcount_add_not_zero(.i=1) Peter Zijlstra
2021-12-10 19:37 ` [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-13 12:15 ` [PATCH v2 10/9] atomic: Document the atomic_{}_overflow() functions Peter Zijlstra
2021-12-13 12:20 ` [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-13 14:42   ` Marco Elver
2021-12-13 16:11     ` Peter Zijlstra

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