Re: [PATCH 09/38] svc: Add a transport function that checks for write space

[Tom Tucker <tom@opengridcomputing.com>]

On Thu, 2007-12-13 at 16:45 -0500, Chuck Lever wrote:
> On Dec 13, 2007, at 4:33 PM, J. Bruce Fields wrote:
> > On Wed, Dec 12, 2007 at 01:10:17PM -0500, Chuck Lever wrote:
> >> On Dec 11, 2007, at 6:32 PM, Tom Tucker wrote:
> >>> +	required = atomic_read(&svsk->sk_reserved) + serv->sv_max_mesg;
> >>> +	wspace = sk_stream_wspace(svsk->sk_sk);
> >>> +
> >>> +	if (wspace < sk_stream_min_wspace(svsk->sk_sk))
> >>> +		return 0;
> >>> +	if (required * 2 > wspace)
> >>> +		return 0;
> >>
> >> Since "required" is an int, this test can behave differently than  
> >> the one
> >> it replaces.
> >
> > If sk_reserved can approach half of 2^31, for example, then surely we
> > have bigger problems?
> 
> 
> What stops sk_reserved from going negative?

Nothing actively _stops_ it from going negative. Indirectly it is
prevented by the "fact" that sv_max_mesg is always greater than the
amount returned by any read on the socket. If this is not true, then
sk_reserved can go negative. That would occur in svc_recv when the
difference between the amount currently reserved (worst case
sv_max_mesg) and the amount read is subtracted from the amount reserved.
If the amount read were greater than sv_max_mesg then the result would
be negative (both meanings intended).

I could add a BUG_ON and run some tests.

> 
> --
> Chuck Lever
> chuck[dot]lever[at]oracle[dot]com