Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

[Peter Zijlstra]

On Tue, Mar 01, 2016 at 10:32:02PM +0100, Peter Zijlstra wrote:

> >         /*
> >          * Thread-local ABI rseq_seqnum field.
> >          * Updated by the kernel, and read by user-space with
> >          * single-copy atomicity semantics. Aligned on 32-bit.
> >          * Values:
> >          * >= 0: current seqnum for this thread (feature is active).
> >          * -1 (initial value): means the rseq feature is inactive.
> >          * -2: rseq feature is not available.
> >          */
> >         int32_t rseq_seqnum;
> 
> So I really hate that, that makes we have to check for these special
> values whenever we increment the seq count and cannot have it wrap
> naturally.

Also, since it will wrap, uint32_t is more natural, since the whole
signed overflow thing is somewhat undefined in C.