Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 06, 2019 at 02:26:38AM -0400, Gabriel Krisman Bertazi wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> >
> >> +static int futex_wait_multiple(u32 __user *uaddr, unsigned int flags,
> >> +			       u32 count, ktime_t *abs_time)
> >> +{
> >> +	struct futex_wait_block *wb;
> >> +	struct restart_block *restart;
> >> +	int ret;
> >> +
> >> +	if (!count)
> >> +		return -EINVAL;
> >> +
> >> +	wb = kcalloc(count, sizeof(struct futex_wait_block), GFP_KERNEL);
> >> +	if (!wb)
> >> +		return -ENOMEM;
> >> +
> >> +	if (copy_from_user(wb, uaddr,
> >> +			   count * sizeof(struct futex_wait_block))) {
> >> +		ret = -EFAULT;
> >> +		goto out;
> >> +	}
> >
> > I'm thinking we can do away with this giant copy and do it one at a time
> > from the other function, just extend the storage allocated there to
> > store whatever values are still required later.

> I'm not sure I get the suggestion here.  If I understand the code
> correctly, once we do it one at a time, we need to queue_me() each futex
> and then drop the hb lock, before going to the next one. 

So the idea is to reduce to a single allocation; like Thomas also said.
And afaict, we've not yet taken any locks the first time we iterate --
that only does get_futex_key(), the whole __futex_wait_setup() and
queue_me(), comes later, in the second iteration.