From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from neil.brown.name (neil.brown.name [103.29.64.221]) (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 C66372DC761 for ; Wed, 30 Apr 2025 04:05:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=103.29.64.221 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745985903; cv=none; b=brQr+IDv1K9BRvqvygA/KKFTHTEuo8meihXZTOIvRjGIRXuqI3PvKOR8UoVgK6pKtZPNO2GuAXSSV82Fq2irhVAHelkE9foBRvBb4SDhIf7Abv80WXoXhJnm6VKY7uzcT08jiajDIn/WXTxE+1bAg2l/MWmlcYNdgmMIQc5nPYE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745985903; c=relaxed/simple; bh=kR1hRb9xbzlIFgc45wvPBLBUbqZAfGjtLUT0nUGj5g0=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ke9J61sVUgXBONcMmESxF9bkxvfICclnqivlcm6Dka1KMKmOUYEg/t8rjyFRbnL5YpIai+r7YZYNcN+5grFmYAY2/2lWTMIH+v1e10rnFolkH7V5xRlt26mI4gQfSs4YEarRx5L5RUB5miZTiMZ8YOKPHCalIEaeaRGD1zTrf0A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=brown.name; spf=pass smtp.mailfrom=neil.brown.name; arc=none smtp.client-ip=103.29.64.221 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=brown.name Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=neil.brown.name Received: from 196.186.233.220.static.exetel.com.au ([220.233.186.196] helo=home.neil.brown.name) by neil.brown.name with esmtp (Exim 4.95) (envelope-from ) id 1u9ygn-005EYL-38; Wed, 30 Apr 2025 04:04:53 +0000 From: NeilBrown To: Trond Myklebust , Anna Schumaker , Mike Snitzer , =?UTF-8?q?Pali=20Roh=C3=A1r?= , Vincent Mailhol Cc: Chuck Lever , Jeff Layton , linux-nfs@vger.kernel.org Subject: [PATCH 1/6] nfs_localio: use cmpxchg() to install new nfs_file_localio Date: Wed, 30 Apr 2025 14:01:11 +1000 Message-ID: <20250430040429.2743921-2-neil@brown.name> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20250430040429.2743921-1-neil@brown.name> References: <20250430040429.2743921-1-neil@brown.name> Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Rather than using nfs_uuid.lock to protect installing a new ro_file or rw_file, change to use cmpxchg(). Removing the file already uses xchg() so this improves symmetry and also makes the code a little simpler. Also remove the optimisation of not taking the lock, and not removing the nfs_file_localio from the linked list, when both ->ro_file and ->rw_file are already NULL. Given that ->nfs_uuid was not NULL, it is extremely unlikely that neither ->ro_file or ->rw_file is NULL so this optimisation can be of little value and it complicates understanding of the code - why can the list_del_init() be skipped? Finally, move the assignment of NULL to ->nfs_uuid until after the last action on the nfs_file_localio (the list_del_init). As soon as this is NULL a racing nfs_close_local_fh() can bypass all the locking and go on to free the nfs_file_localio, so we must be certain to be finished with it first. Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client") Signed-off-by: NeilBrown --- fs/nfs/localio.c | 11 +++-------- fs/nfs_common/nfslocalio.c | 36 ++++++++++++++++-------------------- 2 files changed, 19 insertions(+), 28 deletions(-) diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c index 5c21caeae075..3674dd86f095 100644 --- a/fs/nfs/localio.c +++ b/fs/nfs/localio.c @@ -279,14 +279,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred, if (IS_ERR(new)) return NULL; /* try to swap in the pointer */ - spin_lock(&clp->cl_uuid.lock); - nf = rcu_dereference_protected(*pnf, 1); - if (!nf) { - nf = new; - new = NULL; - rcu_assign_pointer(*pnf, nf); - } - spin_unlock(&clp->cl_uuid.lock); + nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new))); + if (!nf) + swap(nf, new); rcu_read_lock(); } nf = nfs_local_file_get(nf); diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c index 6a0bdea6d644..d72eecb85ea9 100644 --- a/fs/nfs_common/nfslocalio.c +++ b/fs/nfs_common/nfslocalio.c @@ -285,28 +285,24 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl) return; } - ro_nf = rcu_access_pointer(nfl->ro_file); - rw_nf = rcu_access_pointer(nfl->rw_file); - if (ro_nf || rw_nf) { - spin_lock(&nfs_uuid->lock); - if (ro_nf) - ro_nf = rcu_dereference_protected(xchg(&nfl->ro_file, NULL), 1); - if (rw_nf) - rw_nf = rcu_dereference_protected(xchg(&nfl->rw_file, NULL), 1); - - /* Remove nfl from nfs_uuid->files list */ - RCU_INIT_POINTER(nfl->nfs_uuid, NULL); - list_del_init(&nfl->list); - spin_unlock(&nfs_uuid->lock); - rcu_read_unlock(); + ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL)); + rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL)); - if (ro_nf) - nfs_to_nfsd_file_put_local(ro_nf); - if (rw_nf) - nfs_to_nfsd_file_put_local(rw_nf); - return; - } + spin_lock(&nfs_uuid->lock); + /* Remove nfl from nfs_uuid->files list */ + list_del_init(&nfl->list); + spin_unlock(&nfs_uuid->lock); rcu_read_unlock(); + /* Now we can allow racing nfs_close_local_fh() to + * skip the locking. + */ + RCU_INIT_POINTER(nfl->nfs_uuid, NULL); + + if (ro_nf) + nfs_to_nfsd_file_put_local(ro_nf); + if (rw_nf) + nfs_to_nfsd_file_put_local(rw_nf); + return; } EXPORT_SYMBOL_GPL(nfs_close_local_fh); -- 2.49.0