|
On Tue, Aug 12, 2008 at 12:57:21AM +0200, Sven Wegener wrote:
> Both schedulers have a race condition that happens in the following
> situation:
>
> We have an entry in our table that already has expired according to it's
> last use time. Then we need to schedule a new connection that uses this
> entry.
>
> CPU 1 CPU 2
>
> ip_vs_lblc_schedule()
> ip_vs_lblc_get()
> lock table for read
> find entry
> unlock table
> ip_vs_lblc_check_expire()
> lock table for write
> kfree() expired entry
> unlock table
> return invalid entry
>
> Problem is that we assign the last use time outside of our critical
> region. We can make hitting this race more difficult, if not impossible,
> if we assign the last use time while still holding the lock for reading.
> That gives us six minutes during which it's save to use the entry, which
> should be enough for our use case, as we're going to use it immediately
> and don't keep a long reference to it.
>
> We're holding the lock for reading and not for writing. The last use time
> is an unsigned long, so the assignment should be atomic by itself. And we
> don't care, if some other user sets it to a slightly different value. The
> read_unlock() implies a barrier so that other CPUs see the new last use
> time during cleanup, even if we're just using a read lock.
>
> Other solutions would be: 1) protect the whole ip_vs_lblc_schedule() with
> write_lock()ing the lock, 2) add reference counting for the entries, 3)
> protect each entry with it's own lock. And all are bad for performance.
>
> Comments? Ideas?
Is there a pathological case here if sysctl_ip_vs_lblc_expiration is
set to be very short and we happen to hit ip_vs_lblc_full_check()?
To be honest I think that I like the reference count approach best,
as it seems safe and simple. Is it really going to be horrible
for performance?
If so, I wonder if a workable solution would be to provide a more fine-grained
lock on tbl. Something like the way that ct_read_lock/unlock() works.
--
To unsubscribe from this list: send the line "unsubscribe lvs-devel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
|
