Re: [PATCH 1/2] fs: add support for LOOKUP_NONBLOCK

[Jens Axboe <axboe@kernel.dk>]

On 12/11/20 2:51 PM, Al Viro wrote:
> On Fri, Dec 11, 2020 at 11:50:12AM -0700, Jens Axboe wrote:
> 
>> I could filter on O_TRUNC (and O_CREAT) in the caller from the io_uring
>> side, and in fact we may want to do that in general for RESOLVE_LOOKUP
>> as well.
> 
> You do realize that it covers O_RDWR as well, right?  If the object is on
> a frozen filesystem, mnt_want_write() will block until the thing gets thawed.

I do, current patch does have that handled. I was only referring to the
fact that I don't consider O_TRUNC interesting enough to fold in non-block
support for it, I'm quite happy just letting that be as it is and just
disallow it in the flags directly.

>>> AFAICS, without that part it is pretty much worthless.  And details
>>> of what you are going to do in the missing bits *do* matter - unlike the
>>> pathwalk side (which is trivial) it has potential for being very
>>> messy.  I want to see _that_ before we commit to going there, and
>>> a user-visible flag to openat2() makes a very strong commitment.
>>
>> Fair enough. In terms of patch flow, do you want that as an addon before
>> we do RESOLVE_NONBLOCK, or do you want it as part of the core
>> LOOKUP_NONBLOCK patch?
> 
> I want to understand how it will be done.

Of course. I'll post what I have later, easier to discuss an actual
series of patches.

>> Agree, if we're going bool, we should make it the more usually followed
>> success-on-false instead. And I'm happy to see you drop those
>> likely/unlikely as well, not a huge fan. I'll fold this into what I had
>> for that and include your naming change.
> 
> BTW, I wonder if the compiler is able to figure out that
> 
> bool f(void)
> {
> 	if (unlikely(foo))
> 		return false;
> 	if (unlikely(bar))
> 		return false;
> 	return true;
> }
> 
> is unlikely to return false.  We can force that, obviously (provide an inlined
> wrapper and slap likely() there), but...

Not sure, it _should_, but reality may differ with that guess.

-- 
Jens Axboe