From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH] dcache: Translating dentry into pathname without taking rename_lock Date: Wed, 04 Sep 2013 22:17:04 -0400 Message-ID: <5227E9A0.9020802@hp.com> References: <1378321523-40893-1-git-send-email-Waiman.Long@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Alexander Viro , linux-fsdevel , Linux Kernel Mailing List , "Chandramouleeswaran, Aswin" , "Norton, Scott J" To: Linus Torvalds Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 09/04/2013 05:31 PM, Linus Torvalds wrote: > On Wed, Sep 4, 2013 at 12:05 PM, Waiman Long wrote: >> + rcu_read_unlock(); >> + if (read_seqretry(&rename_lock, seq)) >> + goto restart; > Btw, you have this pattern twice, and while it's not necessarily > incorrect, it's a bit worrisome for performance. The rcu_read_unlock sequence in the middle of prepend_path() is not likely to executed. So it shouldn't affect performance exception for the conditional check. > The rcu_read_unlock() is very possibly going to trigger an immediate > scheduling event, so checking the sequence lock after dropping the > read-lock sounds like it would make it much easier to hit the race > with some rename. I can put read_seqbegin/read_seqretry within the rcu_read_lock/rcu_read_unlock block. However, read_seqbegin() can spin for a while if a rename is in progress. So it depends on which is more important, a shorter RCU critical section at the expense of more retries or vice versa. > I'm also a tiny bit worried about livelocking on the sequence lock in > the presence of lots of renames, so I'm wondering if the locking > should try to approximate what we do for the RCU lookup path: start > off optimistically using just the RCU lock and a sequence point, but > if that fails, fall back to taking the real lock. Maybe using a > counter ("try twice, then get the rename lock for writing") > > Hmm? Yes, I can implement a counter that switch to taking the rename_lock if the count reaches 0. It shouldn't be too hard. That should avoid the possibility of a livelock. Longman