On Monday 19 October 2015 03:22 PM, Peter Zijlstra wrote:
> On Mon, Oct 19, 2015 at 09:46:35AM +0000, Vineet Gupta wrote:
>> 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 don't have issue with adding the syscall per-se. But that comes with it's own
headaches of ABI change - more importantly it requires several things to match,
libc, kernel... It would be easier if change was confined to say perf.
Can we use existing syscall(s) - again this is what our good old pthread library
code did.
static void __pthread_acquire(int * spinlock)
{
int cnt = 0;
struct timespec tm;
READ_MEMORY_BARRIER();
while (testandset(spinlock)) { <---- atomic EXchange
if (cnt < 50) {
sched_yield();
cnt++;
} else {
tm.tv_sec = 0;
tm.tv_nsec = 2000001;
nanosleep(&tm, ((void *)0));
cnt = 0;
}
}
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