From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:42590 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932649AbeCMX7d (ORCPT ); Tue, 13 Mar 2018 19:59:33 -0400 Date: Tue, 13 Mar 2018 23:59:30 +0000 From: Al Viro To: John Ogness Cc: Linus Torvalds , linux-fsdevel , Christoph Hellwig , Thomas Gleixner , Peter Zijlstra , Sebastian Andrzej Siewior , Linux Kernel Mailing List , Eric Biederman Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) Message-ID: <20180313235930.GX30522@ZenIV.linux.org.uk> References: <20180223150928.GC30522@ZenIV.linux.org.uk> <20180223174216.GD30522@ZenIV.linux.org.uk> <20180223201317.GG30522@ZenIV.linux.org.uk> <20180224002248.GH30522@ZenIV.linux.org.uk> <20180225073950.GI30522@ZenIV.linux.org.uk> <87bmgbnhar.fsf_-_@linutronix.de> <20180312191351.GN30522@ZenIV.linux.org.uk> <87zi3bn1on.fsf@linutronix.de> <87lgevn0ss.fsf@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87lgevn0ss.fsf@linutronix.de> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, Mar 13, 2018 at 10:05:55PM +0100, John Ogness wrote: > > + rcu_read_lock(); /* to protect parent */ > > + spin_unlock(&dentry->d_lock); > > + parent = READ_ONCE(dentry->d_parent); > > The preceeding line should be removed. We already have a "parent" from > before we did the most recent trylock(). Nope. We have parent, yes, but it had been fetched outside of rcu_read_lock(). So the object it used to point to might have been already freed and we can't do this: > > + spin_lock(&parent->d_lock); To get rid of that reread we'd need to do this: rcu_read_lock(); parent = dentry->d_parent; if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock))) { rcu_read_unlock(); return true; } spin_unlock(&dentry->d_lock); spin_lock(&parent->d_lock); if (unlikely(parent != dentry->d_parent)) { .... Come to think of that, it might make sense to lift rcu_read_lock() all the way out of that sucker. Objections? Below is the incremental I'd fold into that commit: diff --git a/fs/dcache.c b/fs/dcache.c index f0e73c93182b..0d1dac750c0a 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1000,7 +1000,6 @@ static bool shrink_lock_dentry(struct dentry *dentry) inode = dentry->d_inode; if (inode && unlikely(!spin_trylock(&inode->i_lock))) { - rcu_read_lock(); /* to protect inode */ spin_unlock(&dentry->d_lock); spin_lock(&inode->i_lock); spin_lock(&dentry->d_lock); @@ -1009,16 +1008,14 @@ static bool shrink_lock_dentry(struct dentry *dentry) /* changed inode means that somebody had grabbed it */ if (unlikely(inode != dentry->d_inode)) goto out; - rcu_read_unlock(); } parent = dentry->d_parent; + /* parent will stay allocated until we drop rcu_read_lock */ if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock))) return true; - rcu_read_lock(); /* to protect parent */ spin_unlock(&dentry->d_lock); - parent = READ_ONCE(dentry->d_parent); spin_lock(&parent->d_lock); if (unlikely(parent != dentry->d_parent)) { spin_unlock(&parent->d_lock); @@ -1026,14 +1023,11 @@ static bool shrink_lock_dentry(struct dentry *dentry) goto out; } spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED); - if (likely(!dentry->d_lockref.count)) { - rcu_read_unlock(); + if (likely(!dentry->d_lockref.count)) return true; - } spin_unlock(&parent->d_lock); out: spin_unlock(&inode->i_lock); - rcu_read_unlock(); return false; } @@ -1044,8 +1038,10 @@ static void shrink_dentry_list(struct list_head *list) dentry = list_entry(list->prev, struct dentry, d_lru); spin_lock(&dentry->d_lock); + rcu_read_lock(); if (!shrink_lock_dentry(dentry)) { bool can_free = false; + rcu_read_unlock(); d_shrink_del(dentry); if (dentry->d_lockref.count < 0) can_free = dentry->d_flags & DCACHE_MAY_FREE; @@ -1054,6 +1050,7 @@ static void shrink_dentry_list(struct list_head *list) dentry_free(dentry); continue; } + rcu_read_unlock(); d_shrink_del(dentry); parent = dentry->d_parent; __dentry_kill(dentry);