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 5031C1DE881 for ; Thu, 16 Jan 2025 15:42:59 +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=1737042179; cv=none; b=Sq+3RAZRdc34/Fiu7VhLHpbqVcTSWjVcHxG+n5EZO+rotUZfVs3gtL0uFBFMJB1HnFanGLjlE0VEN5eePGBQ0QyO/0gbAc25GYZOvjvq4pVT26bS3hEtJglra9C1QTeLzQzCp0nJvExgKanxw+ZjN0U+qMVQQJ9AchRxGzHul10= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737042179; c=relaxed/simple; bh=7mdZP+xYz2ozKyhLZC2kjuvvKP91l8E0oLFE7c+nHTs=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Qq3ApKxva7wXetdHDUVNF/oK+hnmfvTKaMF17vp4Nvi/0HU2FbSEmd00tEfiWvm2hQDvItFB4EfZlfgob4SSOlsbtR+CUTsvwucjKwDKrtkeGHXhwC2FOZ3SssvPrt+ZQfrqWkYbHnuZt3odr8RN9Ddl4h89QPE2gWhnJm9aceg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dGV1Rr3d; 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="dGV1Rr3d" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8F70AC4CEE1; Thu, 16 Jan 2025 15:42:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737042178; bh=7mdZP+xYz2ozKyhLZC2kjuvvKP91l8E0oLFE7c+nHTs=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=dGV1Rr3dgdH5wAww4gpjkgCcu0vl9jjCXGmslGUFa0jaTP/8g357HnZ8qnXNJdFkN KSYoTUX78OeJmMQDF648yxC5UpoyAXdKubojxSj/n9HTnZAORjJ65POUhSOOSEIKCW 4t0LPg93stV0bj+bPFwzcKi/TswdsESwONT9IvryGiHSYzjk5f3qrJZ9MAdzDNcXH+ KDHSxesJ+zi/JZ6X4Xdz64IewEFDzcuoZu71KGeqweYchPlkeCAl5OU/gpnALoDutW zu2yCoSeY0EPfHhh7a69nrmbKweWwEaCi5egyqfCcgJR+BPJ2ZDymIx/IKjd6T9bZR 89tYU8Xc+2rKQ== Message-ID: <9757da07ce21d1c1275c637ae49cbe69a9c83a71.camel@kernel.org> Subject: Re: [PATCH 1/3] llist: add ability to remove a particular entry from the list From: Jeff Layton To: Chuck Lever , Olga Kornievskaia Cc: linux-nfs@vger.kernel.org Date: Thu, 16 Jan 2025 10:42:57 -0500 In-Reply-To: References: <20250115232406.44815-1-okorniev@redhat.com> <20250115232406.44815-2-okorniev@redhat.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV 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wrote: > > >=20 > > > On 1/15/25 6:24 PM, Olga Kornievskaia wrote: > > > > nfsd stores its network transports in a lwq (which is a lockless li= st) > > > > llist has no ability to remove a particular entry which nfsd needs > > > > to remove a listener thread. > > >=20 > > > Note that scripts/get_maintainer.pl says that the maintainer of this > > > file is: > > >=20 > > > linux-kernel@vger.kernel.org > > >=20 > > > so that address should appear on the cc: list of this series. So > > > should the list of reviewers for NFSD that appear in MAINTAINERS. > >=20 > > I will resend and include the mentioned list. > >=20 > > > ISTR Neil found the same gap in the llist API. I don't think it's > > > possible to safely remove an item from the middle of an llist. Much > > > of the complexity of the current svc thread scheduler is due to this > > > complication. > > >=20 > > > I think you will need to mark the listener as dead and find some > > > way of safely dealing with it later. > >=20 > > A listener can only be removed if there are no active threads. This > > code in nfsd_nl_listener_set_doit() won't allow it which happens > > before we are manipulating the listener: > > /* For now, no removing old sockets while server is running */ > > if (serv->sv_nrthreads && !list_empty(&permsocks)) { > > list_splice_init(&permsocks, &serv->sv_permsocks); > > spin_unlock_bh(&serv->sv_lock); > > err =3D -EBUSY; > > goto out_unlock_mtx; > > } >=20 > Got it. >=20 > I'm splitting hairs, but this seems like a special case that might be > true only for NFSD and could be abused by other API consumers. >=20 > At the least, the opening block comment in llist.h should be updated > to include del_entry in the locking table. >=20 > I would be more comfortable with a solution that works in alignment with > the current llist API, but if others are fine with this solution, then I > won't object strenuously. >=20 > (And to be clear, I agree that there is a bug in set_doit() that needs > to be addressed quickly -- it's the specific fix that I'm queasy about). >=20 Yeah, it would be good to address it quickly since you can crash the box with this right now. I'm not thrilled with adding the llist_del_entry() footgun either, but it should work. Another approach we could consider instead would be to delay queueing all of these sockets to the lwq until after the sv_permsocks list is settled. You could even just queue up the whole sv_permsocks list at the end of this. If there's no work there, then there's no real harm. That is a bit more surgery, however, since you'd have to lift svc_xprt_received() handling out of svc_xprt_create_from_sa(). >=20 > > > > Suggested-by: Jeff Layton > > > > Signed-off-by: Olga Kornievskaia > > > > --- > > > > include/linux/llist.h | 36 ++++++++++++++++++++++++++++++++++++ > > > > 1 file changed, 36 insertions(+) > > > >=20 > > > > diff --git a/include/linux/llist.h b/include/linux/llist.h > > > > index 2c982ff7475a..fe6be21897d9 100644 > > > > --- a/include/linux/llist.h > > > > +++ b/include/linux/llist.h > > > > @@ -253,6 +253,42 @@ static inline bool __llist_add(struct llist_no= de *new, struct llist_head *head) > > > > return __llist_add_batch(new, new, head); > > > > } > > > >=20 > > > > +/** > > > > + * llist_del_entry - remove a particular entry from a lock-less li= st > > > > + * @head: head of the list to remove the entry from > > > > + * @entry: entry to be removed from the list > > > > + * > > > > + * Walk the list, find the given entry and remove it from the list= . >=20 > The above sentence repeats the comments in the code and the code itself. > It visually obscures the next, much more important, sentence. >=20 >=20 > > > > + * The caller must ensure that nothing can race in and change the > > > > + * list while this is running. > > > > + * > > > > + * Returns true if the entry was found and removed. > > > > + */ > > > > +static inline bool llist_del_entry(struct llist_head *head, struct= llist_node *entry) > > > > +{ > > > > + struct llist_node *pos; > > > > + > > > > + if (!head->first) > > > > + return false; >=20 > if (llist_empty()) ? >=20 >=20 > > > > + > > > > + /* Is it the first entry? */ > > > > + if (head->first =3D=3D entry) { > > > > + head->first =3D entry->next; > > > > + entry->next =3D entry; > > > > + return true; >=20 > llist_del_first() or even llist_del_first_this() >=20 > Basically I would avoid open-coding this logic and use the existing > helpers where possible. The new code doesn't provide memory release > semantics that would ensure the next access of the llist sees these > updates. >=20 >=20 > > > > + } > > > > + > > > > + /* Find it in the list */ > > > > + llist_for_each(head->first, pos) { > > > > + if (pos->next =3D=3D entry) { > > > > + pos->next =3D entry->next; > > > > + entry->next =3D entry; > > > > + return true; > > > > + } > > > > + } > > > > + return false; > > > > +} > > > > + > > > > /** > > > > * llist_del_all - delete all entries from lock-less list > > > > * @head: the head of lock-less list to delete all entries > > >=20 > > >=20 > > > -- > > > Chuck Lever > > >=20 > >=20 >=20 >=20 --=20 Jeff Layton