From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta1.migadu.com (out-172.mta1.migadu.com [95.215.58.172]) (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 7707C2F3C2B for ; Mon, 13 Oct 2025 08:26:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760343991; cv=none; b=fkTTblYuL6UyStkJlKADNA+HOaoSLw2G5jKymLXGIWXF8tcbmS/w4PktPVytwpCt2o+1j58vzSE6jCLYuXh1o78cGL+/Y34gvqtesXJJxhwoQrr/pLDuwec83QsOvLUh7YzYZtOfuos5PZz249fVKpS8AzVu98zMMT2HP6PGrsI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760343991; c=relaxed/simple; bh=8Di6RmvMVBX6UKr7C9RsFJsMYdOptVnUpeyEgXBDX30=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=s5q3uu77hve3VolXNBUiwOhXmzECukSZBDD43TYqPQJokE/MJFaipPKjfbxJJc3W93l8nszZVgSfUVSl1eyQFwMg/f3nb6rV6eutwUf0iD9400fdeM8AbdMGOjUZPUo8oXdiZQbN2GH29HtrJl3DUCc0y1Lj5Q8Ez6ZICWaHnPM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=PX6vUgyU; arc=none smtp.client-ip=95.215.58.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="PX6vUgyU" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760343987; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=2x24+Ru0v8QnYj9tz/Et7AyGhyuwGOuCIi1S8FoGdpY=; b=PX6vUgyU7O4Kh9AjbSxGAhqgTYFCCBHOI+4DHFqW8C1FCsZeBFQFXTKOzXfg7LpVf9YxM7 pCwE3qF54GgX4B5dYiLijtelC9ilJnTQTofazOEERshhY1NYNTIur951nlwerBy4M17fLC bnW0R3mCFiMQJdKgCwzN7n8HSw14S98= Date: Mon, 13 Oct 2025 16:25:37 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH net-next v7 1/3] rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu() To: Eric Dumazet Cc: kuniyu@google.com, "Paul E. McKenney" , kerneljasonxing@gmail.com, davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org, Xuanqiang Luo , Frederic Weisbecker , Neeraj Upadhyay References: <20250926074033.1548675-1-xuanqiang.luo@linux.dev> <20250926074033.1548675-2-xuanqiang.luo@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: luoxuanqiang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2025/10/13 15:31, Eric Dumazet 写道: > On Fri, Sep 26, 2025 at 12:41 AM wrote: >> From: Xuanqiang Luo >> >> Add two functions to atomically replace RCU-protected hlist_nulls entries. >> >> Keep using WRITE_ONCE() to assign values to ->next and ->pprev, as >> mentioned in the patch below: >> commit efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for >> rculist_nulls") >> commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for >> hlist_nulls") >> >> Signed-off-by: Xuanqiang Luo >> --- >> include/linux/rculist_nulls.h | 59 +++++++++++++++++++++++++++++++++++ >> 1 file changed, 59 insertions(+) >> >> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h >> index 89186c499dd4..c26cb83ca071 100644 >> --- a/include/linux/rculist_nulls.h >> +++ b/include/linux/rculist_nulls.h >> @@ -52,6 +52,13 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n) >> #define hlist_nulls_next_rcu(node) \ >> (*((struct hlist_nulls_node __rcu __force **)&(node)->next)) >> >> +/** >> + * hlist_nulls_pprev_rcu - returns the dereferenced pprev of @node. >> + * @node: element of the list. >> + */ >> +#define hlist_nulls_pprev_rcu(node) \ >> + (*((struct hlist_nulls_node __rcu __force **)(node)->pprev)) >> + >> /** >> * hlist_nulls_del_rcu - deletes entry from hash list without re-initialization >> * @n: the element to delete from the hash list. >> @@ -152,6 +159,58 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n) >> n->next = (struct hlist_nulls_node *)NULLS_MARKER(NULL); >> } >> >> +/** >> + * hlist_nulls_replace_rcu - replace an old entry by a new one >> + * @old: the element to be replaced >> + * @new: the new element to insert >> + * >> + * Description: >> + * Replace the old entry with the new one in a RCU-protected hlist_nulls, while >> + * permitting racing traversals. >> + * >> + * The caller must take whatever precautions are necessary (such as holding >> + * appropriate locks) to avoid racing with another list-mutation primitive, such >> + * as hlist_nulls_add_head_rcu() or hlist_nulls_del_rcu(), running on this same >> + * list. However, it is perfectly legal to run concurrently with the _rcu >> + * list-traversal primitives, such as hlist_nulls_for_each_entry_rcu(). >> + */ >> +static inline void hlist_nulls_replace_rcu(struct hlist_nulls_node *old, >> + struct hlist_nulls_node *new) >> +{ >> + struct hlist_nulls_node *next = old->next; >> + >> + WRITE_ONCE(new->next, next); >> + WRITE_ONCE(new->pprev, old->pprev); > I do not think these two WRITE_ONCE() are needed. > > At this point new is not yet visible. > > The following rcu_assign_pointer() is enough to make sure prior > writes are committed to memory. Dear Eric, I’m quoting your more detailed explanation from the other patch [0], thank you for that! However, regarding new->next, if the new object is allocated with SLAB_TYPESAFE_BY_RCU, would we still encounter the same issue as in commit efd04f8a8b45 (“rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls”)? Also, for the WRITE_ONCE() assignments to ->pprev introduced in commit 860c8802ace1 (“rcu: Use WRITE_ONCE() for assignments to ->pprev for hlist_nulls”) within hlist_nulls_add_head_rcu(), is that also unnecessary? [0]: https://lore.kernel.org/all/CANn89iKQM=4wjCLxpg-m3jYoUm=rsSk68xVLN2902di2+FkSFg@mail.gmail.com/ Thanks! >> + rcu_assign_pointer(hlist_nulls_pprev_rcu(new), new); >> + if (!is_a_nulls(next)) >> + WRITE_ONCE(next->pprev, &new->next); >> +} >> + >> +/** >> + * hlist_nulls_replace_init_rcu - replace an old entry by a new one and >> + * initialize the old >> + * @old: the element to be replaced >> + * @new: the new element to insert >> + * >> + * Description: >> + * Replace the old entry with the new one in a RCU-protected hlist_nulls, while >> + * permitting racing traversals, and reinitialize the old entry. >> + * >> + * Note: @old must be hashed. >> + * >> + * The caller must take whatever precautions are necessary (such as holding >> + * appropriate locks) to avoid racing with another list-mutation primitive, such >> + * as hlist_nulls_add_head_rcu() or hlist_nulls_del_rcu(), running on this same >> + * list. However, it is perfectly legal to run concurrently with the _rcu >> + * list-traversal primitives, such as hlist_nulls_for_each_entry_rcu(). >> + */ >> +static inline void hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old, >> + struct hlist_nulls_node *new) >> +{ >> + hlist_nulls_replace_rcu(old, new); >> + WRITE_ONCE(old->pprev, NULL); >> +} >> + >> /** >> * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type >> * @tpos: the type * to use as a loop cursor. >> -- >> 2.25.1 >>