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 5CC681D9D6A; Wed, 4 Sep 2024 13:54:07 +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=1725458047; cv=none; b=UYzSa/473KzvYz/wniFDzx7sawFCsCGIi1OBxXsKlaKCeU1PPx9wRgF4wYk/fdeSiM2cutZGy1BDZPtruUmOjVowP2hBI5aFwFiba2ENudxO7Me2E7uTM/0HJNPlcaXgaIeKZ35whm/0UCk2/QxJ/T/KmREho8o+oVWf7oh0l+I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725458047; c=relaxed/simple; bh=7a7xpDgjncCUy1Jz1cjuP7kcdeOBBt0yxKxcWzP3+y8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=rzXyPfx7kU9cbAl95Xq15td9TwU87zG4p0GVeFMHjF7KmuK+SIqEiLUqi6do/k5Nsrsu11MVhvYwoopgaC22660T1TbFOG4Ekkgo4Ay0lUpjwcaYHXWD3WHPPpwuWQ1weSQXnTan71dA3mdjsOeXaNv5kkTZAF5iR7i2uOS50dE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=odrkYU69; 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="odrkYU69" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 488EBC4CEC2; Wed, 4 Sep 2024 13:54:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1725458046; bh=7a7xpDgjncCUy1Jz1cjuP7kcdeOBBt0yxKxcWzP3+y8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=odrkYU69+vpwTKtktqLByC1cvtL//XH9hjdbtM95A5KXBXBGm1qkEgXa27c6KToje zlZJZvq3Ytf9JJySCJcWoEkZSzM17Efzychuq8/v4TR/cP9QT2xMnDRKhJn+OPKvO2 n524gMWlJBmKUcMP3UfDU1DMlrUvvzPJVvFyEn8GzeHfXVoTtZPOo1mA2Z57BwEuOi UalOjRQVcdOvXZwH8mRNZLFdfPrge0NBTV302fRYx7yTHKwsdju8/K25ejLHaXFR4/ GLPqEnqkt0qCOKT7P6+8Cd7A7YJ2HiYFiw8/BlWGd20a6pOARaIFcBv+UdKb8h8gQQ e9QBWK6kkUFtw== Message-ID: <86405a95e604281c4a14d1787bf91b02dbc2b115.camel@kernel.org> Subject: Re: [PATCH v15 16/26] nfsd: add LOCALIO support From: Jeff Layton To: NeilBrown , Chuck Lever III Cc: Mike Snitzer , Linux NFS Mailing List , Anna Schumaker , Trond Myklebust , "linux-fsdevel@vger.kernel.org" Date: Wed, 04 Sep 2024 09:54:04 -0400 In-Reply-To: <172542610641.4433.9213915589635956986@noble.neil.brown.name> References: <>, <67405117-1C08-4CA9-B0CE-743DFC7BCE3F@oracle.com> , <172540270112.4433.6741926579586461095@noble.neil.brown.name> <172542610641.4433.9213915589635956986@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, 2024-09-04 at 15:01 +1000, NeilBrown wrote: > On Wed, 04 Sep 2024, NeilBrown wrote: > >=20 > > I agree that dropping and reclaiming a lock is an anti-pattern and in > > best avoided in general. I cannot see a better alternative in this > > case. >=20 > It occurred to me what I should spell out the alternate that I DO see so > you have the option of disagreeing with my assessment that it isn't > "better". >=20 > We need RCU to call into nfsd, we need a per-cpu ref on the net (which > we can only get inside nfsd) and NOT RCU to call > nfsd_file_acquire_local(). >=20 > The current code combines these (because they are only used together) > and so the need to drop rcu.=20 >=20 > I thought briefly that it could simply drop rcu and leave it dropped > (__releases(rcu)) but not only do I generally like that LESS than > dropping and reclaiming, I think it would be buggy. While in the nfsd > module code we need to be holding either rcu or a ref on the server else > the code could disappear out from under the CPU. So if we exit without > a ref on the server - which we do if nfsd_file_acquire_local() fails - > then we need to reclaim RCU *before* dropping the ref. So the current > code is slightly buggy. >=20 > We could instead split the combined call into multiple nfs_to > interfaces. >=20 > So nfs_open_local_fh() in nfs_common/nfslocalio.c would be something > like: >=20 > rcu_read_lock(); > net =3D READ_ONCE(uuid->net); > if (!net || !nfs_to.get_net(net)) { > rcu_read_unlock(); > return ERR_PTR(-ENXIO); > } > rcu_read_unlock(); > localio =3D nfs_to.nfsd_open_local_fh(....); > if (IS_ERR(localio)) > nfs_to.put_net(net); > return localio; >=20 > So we have 3 interfaces instead of 1, but no hidden unlock/lock. >=20 > As I said, I don't think this is a net win, but reasonable people might > disagree with me. >=20 I considered a few alternate designs here as well, and came to the same conclusion. This interface is ugly, but it's not materially worse than the alternatives. I think we just have to document this well, and deal with the ugliness. Luckily most of the gory details are managed inside nfsd and the nfs_common functions so the caller (nfs) shouldn't have to deal with the complex locking. One thing that might be good if we're sticking with this code, is a __might_sleep() at the top of nfs_open_local_fh function in nfs_common. That should help ensure that no one tries to call it with the rcu_read_lock() held (which is the main danger here). --=20 Jeff Layton