From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AB3EA1FCFD6; Mon, 27 Jan 2025 14:09:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737986966; cv=none; b=iq36P8NFpRm5Vme24WgSnRGk/juIUMfZItEl0yR99e/L3qC4qCfVN9GuybGDT1egD4PUZuvxEun+14mDcLvjAFGWmKT5kj7ikmgl2TYVCRqs7x86sSNogazGIC4J50MLPbImlYcQmoG7vh0Pk8ZjkRxS0BOqqyrJg133X9ArUrA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737986966; c=relaxed/simple; bh=Rz6meftNFWIKeSBHX9X0i9FyWdEWeAzsHOwEmNXnLVo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FskN18sV9NhWZN5VcgjW00oXTay3FjHmLwEn4Omns4kwnlRQRXCvJxY++WrYMcyp/rYxqtruYCeDcXETenhUjUMBydcZ8/6BhJ2YTR+/NuGRd9cjydDzYL6HkajJGhItiTdp/EScGU9NdGL3RcdRdB+ztBTzxWtkPVjUlQHkrfY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PsYVb50t; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PsYVb50t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2894EC4CED2; Mon, 27 Jan 2025 14:09:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737986966; bh=Rz6meftNFWIKeSBHX9X0i9FyWdEWeAzsHOwEmNXnLVo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PsYVb50tmoIzHQ8LPsNtS6QOOFCTm7a45GJ+NKRDdX9fNeEdF2ssy+DhgJVjZ5Na5 dHcLhlj78j5Uxup6pN+BaZW7kDnoL3bjE4Dm8IYFh+EFlo8ytKigkZsxxEkqWetoc2 ImJSh2tqlyO2CDXVfwfHk76IFlf8oSj7Tr0zqVpQ0QmDaz6rzskGEglR+u1qfzuZXO bgRpsOZyB5GHnqjSV7F/g3Hit7qNCm6x4p9bnP20pT2qAg9y82dPWuGxOjlePGGk9n +FQi3j3kTTaZ+h+Ix9z4/diG4SaucBACHB2YO8xh3fMNpn49SQ3nlT+e6W6PX7Ze9a IiOcuPshSze5w== Date: Mon, 27 Jan 2025 14:09:16 +0000 From: Will Deacon To: Peter Zijlstra Cc: Suren Baghdasaryan , boqun.feng@gmail.com, mark.rutland@arm.com, Mateusz Guzik , akpm@linux-foundation.org, willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, david.laight.linux@gmail.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, richard.weiyang@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH] refcount: Strengthen inc_not_zero() Message-ID: <20250127140915.GA25672@willie-the-truck> References: <20250111042604.3230628-1-surenb@google.com> <20250111042604.3230628-12-surenb@google.com> <20250115104841.GX5388@noisy.programming.kicks-ass.net> <20250115111334.GE8385@noisy.programming.kicks-ass.net> <20250115160011.GG8385@noisy.programming.kicks-ass.net> <20250117154135.GA17569@willie-the-truck> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250117154135.GA17569@willie-the-truck> User-Agent: Mutt/1.10.1 (2018-07-13) On Fri, Jan 17, 2025 at 03:41:36PM +0000, Will Deacon wrote: > On Wed, Jan 15, 2025 at 05:00:11PM +0100, Peter Zijlstra wrote: > > On Wed, Jan 15, 2025 at 12:13:34PM +0100, Peter Zijlstra wrote: > > > > > Notably, it means refcount_t is entirely unsuitable for anything > > > SLAB_TYPESAFE_BY_RCU, since they all will need secondary validation > > > conditions after the refcount succeeds. > > > > > > And this is probably fine, but let me ponder this all a little more. > > > > Even though SLAB_TYPESAFE_BY_RCU is relatively rare, I think we'd better > > fix this, these things are hard enough as they are. > > > > Will, others, wdyt? > > We should also update the Documentation (atomic_t.txt and > refcount-vs-atomic.rst) if we strengthen this. > > > --- > > Subject: refcount: Strengthen inc_not_zero() > > > > For speculative lookups where a successful inc_not_zero() pins the > > object, but where we still need to double check if the object acquired > > is indeed the one we set out to aquire, needs this validation to happen > > *after* the increment. > > > > Notably SLAB_TYPESAFE_BY_RCU is one such an example. > > > > Signed-off-by: Peter Zijlstra (Intel) > > --- > > include/linux/refcount.h | 15 ++++++++------- > > 1 file changed, 8 insertions(+), 7 deletions(-) > > > > diff --git a/include/linux/refcount.h b/include/linux/refcount.h > > index 35f039ecb272..340e7ffa445e 100644 > > --- a/include/linux/refcount.h > > +++ b/include/linux/refcount.h > > @@ -69,9 +69,10 @@ > > * its the lock acquire, for RCU/lockless data structures its the dependent > > * load. > > * > > - * Do note that inc_not_zero() provides a control dependency which will order > > - * future stores against the inc, this ensures we'll never modify the object > > - * if we did not in fact acquire a reference. > > + * Do note that inc_not_zero() does provide acquire order, which will order > > + * future load and stores against the inc, this ensures all subsequent accesses > > + * are from this object and not anything previously occupying this memory -- > > + * consider SLAB_TYPESAFE_BY_RCU. > > * > > * The decrements will provide release order, such that all the prior loads and > > * stores will be issued before, it also provides a control dependency, which > > @@ -144,7 +145,7 @@ bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp) > > do { > > if (!old) > > break; > > - } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i)); > > + } while (!atomic_try_cmpxchg_acquire(&r->refs, &old, old + i)); > > Hmm, do the later memory accesses need to be ordered against the store > part of the increment or just the read? If it's the former, then I don't > think that _acquire is sufficient -- accesses can still get in-between > the read and write parts of the RmW. I dug some more into this at the end of last week. For the SLAB_TYPESAFE_BY_RCU where we're racing inc_not_zero() with dec_and_test(), then I think using _acquire() above is correct as the later references can only move up into the critical section in the case that we successfully obtained a reference. However, if we're going to make the barriers implicit in the refcount operations here then I think we also need to do something on the producer side for when the object is re-initialised after being destroyed and allocated again. I think that would necessitate release ordering for refcount_set() so that whatever allows the consumer to validate the object (e.g. sequence number) is published *before* the refcount. Will