On 09/04/2013 05:31 PM, Linus Torvalds wrote:
On Wed, Sep 4, 2013 at 12:05 PM, Waiman Long<Waiman.Long@xxxxxx> 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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