From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta0.migadu.com (out-170.mta0.migadu.com [91.218.175.170]) (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 6FEAB149E17 for ; Wed, 17 Sep 2025 03:27:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758079660; cv=none; b=TyzJi/c4OBREQvRmAlNS+r8j663c9AHXuhKH5JxufxSDKbOZM/5Ke6ggzBmr3SJ+sWH9n2RvDICGndanQ9EaxaJmmXris6A5EUbCjuOXThvAhTKwBSnjrxM/fLiAZeY/qiIlX329bxMSEVkV8rOkM57LRosvImiJ9nleDCvV9/M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758079660; c=relaxed/simple; bh=BjOB9OTEOaJg8z1GF4WL0cjEzqgJDbA4mOw2Xod+D2M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PHTAgnG3thhq0pg4bMYRRKH4k4aL0D6pIFJ6m4DGRlrqM3XO+Xj1kuyCzriwx6jg+GGDW7ihfUKKmXZcyCUgbWHhk4sxbIWgEFnz66bkDcDJ5j/XaQT9YGCh5GycCxpofozXRxfX6x6cVri+SR/LlQLfBbZv76YPCH0H6p4rBEY= 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=pcf+jLaC; arc=none smtp.client-ip=91.218.175.170 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="pcf+jLaC" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758079655; 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=4r1/YY5rtUoZT85csUT78Q834I/tkilYaiEqzCsfLH4=; b=pcf+jLaC9f1hRNvb+tgR5FnCxFOoBRymlecRGAe6A2W7K3IkeE+xL2AycPuE6M4Q3vpd97 aC06NglDQ9u5GnmkN6z8jhz6bhomLe+aG2WsXhVANvWC6aX8iXlLMqvR1fJHfTXEjpH8L/ MCB1r3zCb7FZ9V5Nt76Epq5Ek5pgPVg= Date: Wed, 17 Sep 2025 11:26:55 +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 v3 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu() To: Kuniyuki Iwashima Cc: edumazet@google.com, kerneljasonxing@gmail.com, davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org, Xuanqiang Luo References: <20250916103054.719584-1-xuanqiang.luo@linux.dev> <20250916103054.719584-2-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 2025/9/17 02:58, Kuniyuki Iwashima 写道: > On Tue, Sep 16, 2025 at 3:31 AM wrote: >> From: Xuanqiang Luo >> >> Add two functions to atomically replace RCU-protected hlist_nulls entries. >> >> Signed-off-by: Xuanqiang Luo >> --- >> include/linux/rculist_nulls.h | 61 +++++++++++++++++++++++++++++++++++ >> 1 file changed, 61 insertions(+) >> >> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h >> index 89186c499dd4..8ed604f65a3e 100644 >> --- a/include/linux/rculist_nulls.h >> +++ b/include/linux/rculist_nulls.h >> @@ -152,6 +152,67 @@ 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; >> + >> + new->next = next; Do we need to use WRITE_ONCE() here, as mentioned in efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls")? I am more inclined to think that it is necessary. >> + WRITE_ONCE(new->pprev, old->pprev); > As you don't use WRITE_ONCE() for ->next, the new node must > not be published yet, so WRITE_ONCE() is unnecessary for ->pprev > too. I noticed that point. My understanding is that using WRITE_ONCE() for new->pprev follows the approach in hlist_replace_rcu() to match the READ_ONCE() in hlist_nulls_unhashed_lockless() and hlist_unhashed_lockless(). > >> + rcu_assign_pointer(*(struct hlist_nulls_node __rcu **)new->pprev, new); >> + if (!is_a_nulls(next)) >> + WRITE_ONCE(new->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. >> + * >> + * Return: true if the old entry was hashed and was replaced successfully, false >> + * otherwise. >> + * >> + * Note: hlist_nulls_unhashed() on the old node returns true after this. >> + * It is useful for RCU based read lockfree traversal if the writer side must >> + * know if the list entry is still hashed or already unhashed. >> + * >> + * 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 bool hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old, >> + struct hlist_nulls_node *new) >> +{ >> + if (!hlist_nulls_unhashed(old)) { > As mentioned in v1, this check is redundant. Apologies for bringing this up again. My understanding is that replacing a node requires checking if the old node is unhashed. If so, we need a return value to inform the caller that the replace operation would fail. > >> + __hlist_nulls_replace_rcu(old, new); >> + WRITE_ONCE(old->pprev, NULL); >> + return true; >> + } >> + return false; >> +} >> + >> /** >> * 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 >>