RE: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount_t

[David Laight]

From: Herbert Xu
> Sent: 20 March 2017 13:16
> On Mon, Mar 20, 2017 at 11:39:37AM +0100, Peter Zijlstra wrote:
> >
> > Can we at least give a benchmark and have someone run numbers? We should
> > be able to quantify these things.
> 
> Do you realise how many times this thing gets hit at 10Gb/s or
> higher? Anyway, since you're proposing this change you should
> demonstrate that it does not cause a performance regression.

What checks does refcnt_t actually do?

An extra decrement is hard to detect since the item gets freed early.
I guess making the main 'allocate/free' code hold (say) 64k references
would give some leeway for extra decrements.

An extra increment will be detected when the count eventually wraps.
Unless the error is in a very common path that won't happen for a long time.

On x86 the cpu flags from the 'lock inc/dec' could be used to reasonably
cheaply detect errors - provided you actually generate a forwards branch.

Otherwise having a common, but not every packet, code path verify that the
reference count is 'sane' would give reasonable coverage.

	David