From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964831AbeBMNQh (ORCPT ); Tue, 13 Feb 2018 08:16:37 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:32974 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964775AbeBMNQg (ORCPT ); Tue, 13 Feb 2018 08:16:36 -0500 Date: Tue, 13 Feb 2018 14:16:33 +0100 From: Peter Zijlstra To: Will Deacon Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Al Viro Subject: Re: [PATCH] fs: dcache: Avoid livelock between d_alloc_parallel and __d_add Message-ID: <20180213131633.GI25201@hirez.programming.kicks-ass.net> References: <1518526731-26546-1-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1518526731-26546-1-git-send-email-will.deacon@arm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 13, 2018 at 12:58:51PM +0000, Will Deacon wrote: > If d_alloc_parallel runs concurrently with __d_add, it is possible for > d_alloc_parallel to continuously retry whilst i_dir_seq has been > incremented to an odd value by __d_add: > > CPU0: > __d_add > n = start_dir_add(dir); > cmpxchg(&dir->i_dir_seq, n, n + 1) == n > > CPU1: > d_alloc_parallel > retry: > seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1; > hlist_bl_lock(b); > bit_spin_lock(0, (unsigned long *)b); // Always succeeds > > CPU0: > __d_lookup_done(dentry) > hlist_bl_lock > bit_spin_lock(0, (unsigned long *)b); // Never succeeds > > CPU1: > if (unlikely(parent->d_inode->i_dir_seq != seq)) { > hlist_bl_unlock(b); > goto retry; > } > > Since the simple bit_spin_lock used to implement hlist_bl_lock does not And cannot, a single bit is just not enough state. > provide any fairness guarantees, then CPU1 can starve CPU0 of the lock > and prevent it from reaching end_dir_add(dir), therefore CPU1 cannot > exit its retry loop because the sequence number always has the bottom > bit set. > > This patch resolves the livelock by not taking hlist_bl_lock in > d_alloc_parallel if the sequence counter is odd, since any subsequent > masked comparison with i_dir_seq will fail anyway. > Acked-by: Peter Zijlstra (Intel) > Cc: Al Viro > Signed-off-by: Will Deacon > --- > fs/dcache.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/fs/dcache.c b/fs/dcache.c > index 7c38f39958bc..b243deec298c 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -2474,7 +2474,7 @@ struct dentry *d_alloc_parallel(struct dentry *parent, > > retry: > rcu_read_lock(); > - seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1; > + seq = smp_load_acquire(&parent->d_inode->i_dir_seq); > r_seq = read_seqbegin(&rename_lock); > dentry = __d_lookup_rcu(parent, name, &d_seq); > if (unlikely(dentry)) { > @@ -2495,6 +2495,12 @@ struct dentry *d_alloc_parallel(struct dentry *parent, > rcu_read_unlock(); > goto retry; > } > + > + if (unlikely(seq & 1)) { > + rcu_read_unlock(); > + goto retry; > + } > + > hlist_bl_lock(b); > if (unlikely(parent->d_inode->i_dir_seq != seq)) { Also, should that not read: if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) { I mean, load-tearing can only result in additional failure, but still. > hlist_bl_unlock(b);