[$] Avoiding blocking file-name lookups

As a general rule, when one attempts to open a file with a system call like
openat2(),
the expectation is that the call will not return until the job is done.
But there are times where the desire to open the file is conditional on
being able to open it immediately, without blocking. Linux has never
supported that mode well, but that may be about to change with this
patch set from Jens Axboe.