On Monday 19 October 2015 03:22 PM, Peter Zijlstra wrote:

On Mon, Oct 19, 2015 at 09:46:35AM +0000, Vineet Gupta wrote:

quoted

quoted

On ARC we could use the atomic EXchange to implement a user space only binary
semaphore - these atomic ops will be small duration so it is OK to spin wait for a
little bit. That's how the old pthread library worked for ARC w/o any atomic support.

That has the obvious problem of lock-holder-preemption and the horrible
performance issues that result from that.

I think the syscall at least has deterministic behaviour, whereas that
userspace spin loop has this abysmal worst case thing.

I agree - we can add that syscall trivially and use it based on build time feature
detection for atomics !