Re: rhashtable: Prevent spurious EBUSY errors on insertion

Phil Sutter
2015-12-03T15:38:09Z

On Thu, Dec 03, 2015 at 08:41:29PM +0800, Herbert Xu wrote:
> On Mon, Nov 30, 2015 at 06:18:59PM +0800, Herbert Xu wrote:
> >
> > OK that's better. I think I see the problem. The test in
> > rhashtable_insert_rehash is racy and if two threads both try
> > to grow the table one of them may be tricked into doing a rehash
> > instead.
> >
> > I'm working on a fix.
>
> OK this patch fixes the EBUSY problem as far as I can tell. Please
> let me know if you still observe EBUSY with it. I'll respond to the
> ENOMEM problem in another email.
>
> ---8<---
> Thomas and Phil observed that under stress rhashtable insertion
> sometimes failed with EBUSY, even though this error should only
> ever been seen when we're under attack and our hash chain length
> has grown to an unacceptable level, even after a rehash.
>
> It turns out that the logic for detecting whether there is an
> existing rehash is faulty. In particular, when two threads both
> try to grow the same table at the same time, one of them may see
> the newly grown table and thus erroneously conclude that it had
> been rehashed. This is what leads to the EBUSY error.
>
> This patch fixes this by remembering the current last table we
> used during insertion so that rhashtable_insert_rehash can detect
> when another thread has also done a resize/rehash. When this is
> detected we will give up our resize/rehash and simply retry the
> insertion with the new table.
>
> Reported-by: Thomas Graf <tgraf@suug.ch>
> Reported-by: Phil Sutter <phil@nwl.cc>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Tested-by: Phil Sutter <phil@nwl.cc>