Messages in this thread | | | Date | Wed, 04 Sep 2013 22:17:04 -0400 | From | Waiman Long <> | Subject | Re: [PATCH] dcache: Translating dentry into pathname without taking rename_lock |
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On 09/04/2013 05:31 PM, Linus Torvalds wrote: > On Wed, Sep 4, 2013 at 12:05 PM, Waiman Long<Waiman.Long@hp.com> 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
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