From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 99952375F87; Fri, 29 May 2026 17:02:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780074179; cv=none; b=pL99ADfxl56t/HEyhoFCXMq409xgfseAq7ELX1cfTmmQT5c7MIJxDFyVMt6ufjjXwzluhZ2Vq4GztpE0KoruVo4ghrIgprhzJu+r3wBwxAnpdND0T3A4yQq/VsHAp+hetHqHRgG6J7uDslOkTGJzGUsDgp69O65ilS+gCj02+M4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780074179; c=relaxed/simple; bh=ZahAwlZka8N7ccizV2LILK9praVIj6iXtpcb4t0XfNA=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=S3MlfOtTuljcrX6ex6LY8ZWjmLPVZNovfDiJVZxcqGW0qgJS1wgSUmMCzVdkLl6EDqImNeNndn7KjBrOX4KFjgOoCRjOUE1HG1fz5mORc37IEbGPub23CV19xpSg4U4PRiGZajpwuDZTZRJva5iwf9t0F3uTFmUCd59ke4cBS9k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XwSrX919; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="XwSrX919" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D1E401F00893; Fri, 29 May 2026 17:02:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780074177; bh=V4AqBYyxvCsHsRn+O7A8zGV8iZdZW9auVshSQf7BkBI=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=XwSrX919BO5R/dADW4xkjwFQL+U6z005nf0hIWo1DgBQaaz+r6dp5rE1y9aIRHQr7 hEY/tt2D6jwRDNFi++5xO8BLkKWlLSOCWAX1ZouLrAB8m4Q+5GSy+GHXlhrVLzCw4X IPBWk21xxsg4RGYz3LY/akreb2mR34OWbzRj7gVYw+sfuATJtH65z3fBYL/qQl2nUw ONAjMouJfkvbrApqyb9xFFlGjQ+Qxym1wMwr7g7htLRjUdJQ+lPLC+hFc5tyMsHbxY Arzu6VZQ0ybxwcBIvmXKfbPO881xqZ18StNYBj+hZfdFsfOe++BVTwyHnVdG1YA/yT jnt2TxkRfGf7A== Message-ID: Subject: Re: [PATCH 03/10] nfsd: serialize nfsd4_end_grace() with atomic test-and-set From: Jeff Layton To: Chuck Lever , Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , "J. Bruce Fields" , Scott Mayhew , Trond Myklebust , Andreas Gruenbacher , Mike Snitzer , Rick Macklem Cc: Chris Mason , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 29 May 2026 13:02:54 -0400 In-Reply-To: References: <20260528-nfsd-fixes-v1-0-e78708eff77d@kernel.org> <20260528-nfsd-fixes-v1-3-e78708eff77d@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-05-29 at 12:05 -0400, Chuck Lever wrote: > On 5/29/26 11:57 AM, Jeff Layton wrote: > > On Fri, 2026-05-29 at 11:38 -0400, Chuck Lever wrote: > > >=20 > > > On Thu, May 28, 2026, at 5:55 PM, Jeff Layton wrote: > > > > From: Chris Mason > > > >=20 > > > > nfsd4_end_grace() guards its drain path with a plain bool: > > > >=20 > > > > if (nn->grace_ended) > > > > return; > > > > nn->grace_ended =3D true; > > > >=20 > > > > The read and the write are independent, and nothing in struct > > > > nfsd_net serializes them. At least two contexts can reach this > > > > code with no lock held: > > > >=20 > > > > laundromat path > > > > laundry_wq kworker > > > > nfs4_laundromat() > > > > nfsd4_end_grace() > > > >=20 > > > > RECLAIM_COMPLETE path > > > > nfsd compound kthread > > > > nfsd4_reclaim_complete() > > > > inc_reclaim_complete() > > > > nfsd4_end_grace() > > > >=20 > > > > Both callers can observe grace_ended =3D=3D false on different CPUs= , > > > > both store true, and both proceed into nfsd4_record_grace_done(), > > > > which invokes the active client_tracking_ops->grace_done callback. > > > > For tracking ops that drain reclaim_str_hashtbl (legacy_tracking_op= s > > > > via nfsd4_recdir_purge_old, and the cld v1+ ops via > > > > nfsd4_cld_grace_done), grace_done calls nfs4_release_reclaim(), > > > > which walks every bucket of reclaim_str_hashtbl with no lock and > > > > calls nfs4_remove_reclaim_record() (list_del + kfree) on each > > > > entry. Two concurrent walkers corrupt the list and double-free > > > > every nfs4_client_reclaim. A concurrent nfsd4_find_reclaim_client(= ) > > > > iterating the same bucket reads through freed memory. > > > >=20 > > > > A third call site exists in nfs4_state_start_net() on the > > > > skip_grace startup path, but it runs under nfsd_mutex before any > > > > client has connected and before the laundromat's first delayed > > > > work fires, so it cannot race with the two callers above. > > > >=20 > > > > Fix by replacing the read/write pair with try_cmpxchg() so exactly > > > > one caller transitions grace_ended from false to true and proceeds > > > > into the drain; the loser returns immediately. bool supports > > > > 1-byte cmpxchg on all supported architectures, and no lock > > > > ordering changes are needed. > > > >=20 > > > > Fixes: 362063a595be ("nfsd: keep a tally of RECLAIM_COMPLETE operat= ions=20 > > > > when using nfsdcld") > > > > Assisted-by: kres:claude-opus-4-7 > > > > Signed-off-by: Chris Mason > > > > --- > > > > fs/nfsd/nfs4state.c | 17 ++++++++++++++--- > > > > 1 file changed, 14 insertions(+), 3 deletions(-) > > > >=20 > > > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c > > > > index f4d12dbcf97b..dc4ac541436f 100644 > > > > --- a/fs/nfsd/nfs4state.c > > > > +++ b/fs/nfsd/nfs4state.c > > > > @@ -7022,12 +7022,23 @@ nfsd4_renew(struct svc_rqst *rqstp, struct= =20 > > > > nfsd4_compound_state *cstate, > > > > static void > > > > nfsd4_end_grace(struct nfsd_net *nn) > > > > { > > > > - /* do nothing if grace period already ended */ > > > > - if (nn->grace_ended) > > > > + bool expected =3D false; > > > > + > > > > + /* > > > > + * nfsd4_end_grace() can be entered concurrently from the > > > > + * laundromat workqueue and from an nfsd compound thread > > > > + * handling RECLAIM_COMPLETE. Without serialization, both > > > > + * callers can observe grace_ended=3D=3Dfalse and proceed into > > > > + * nfsd4_record_grace_done(). For tracking ops whose > > > > + * grace_done drains reclaim_str_hashtbl, that results in > > > > + * list corruption and a double free of every > > > > + * nfs4_client_reclaim entry. Use an atomic test-and-set so > > > > + * exactly one caller proceeds. > > > > + */ > > > > + if (!try_cmpxchg(&nn->grace_ended, &expected, true)) > > > > return; > > > >=20 > > > > trace_nfsd_grace_complete(nn); > > > > - nn->grace_ended =3D true; > > > > /* > > > > * If the server goes down again right now, an NFSv4 > > > > * client will still be allowed to reclaim after it comes back up= , > > > >=20 > > > > --=20 > > > > 2.54.0 > > >=20 > > > Seems like the usual idiom for something like this is an atomic > > > bit op. Perhaps try_cmpxchg on a boolean variable is not going > > > to behave as you expect on every hardware platform. > >=20 > > We just need a single flag here though. try_cmpxchg() had better work > > the same way on every platform or a lot of stuff is FUBAR. Where > > wouldn't it? >=20 > Codex suggests on Hexagon, cmpxchg grabs more than just that boolean. >=20 Ok, I'm convinced. What I think I'll do is add a new flags unsigned long to struct nfsd_net and we can convert most of the bools in that struct to use it instead. --=20 Jeff Layton