On Sat, Mar 19, 2016 at 06:20:24PM -0700, Linus Torvalds wrote:

On Mon, Mar 14, 2016 at 10:17 AM, Benjamin LaHaise [off-list ref] wrote:

quoted

I had some time last week to make an aio openat do what it can in
submit context.  The results are an improvement: when openat is handled
in submit context it completes in about half the time it takes compared
to the round trip via the work queue, and it's not terribly much code
either.

This looks good to me, and I do suspect that any of these aio paths
should strive to have a synchronous vs threaded model. I think that
makes the whole thing much more interesting from a performance
standpoint.

Umm...  You do realize that LOOKUP_RCU in flags does *NOT* guarantee that
it won't block, right?  At the very least one would need to refuse to
fall back on non-RCU mode without a full restart.  Furthermore, vfs_open()
itself can easily block.

So this new LOOKUP flag makes no sense, and it's in the just about _the_
worst place possible for adding special cases with ill-defined semantics -
do_last() is already far too convoluted and needs untangling, not adding
half-assed kludges.

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