On Thu, Jun 28, 2018 at 02:11:17PM -0700, Linus Torvalds wrote: > Christoph, do you have a test program for IOCB_CMD_POLL and what it's > actually supposed to do? https://pagure.io/libaio/c/9c6935e81854d1585bbfa48c35b185849d746864?branch=aio-poll is the actual test in libaio. In addition to that the seastar library actually has a real life user. But given that is c++ with all modern bells and whistles you'll probably have an as hard time as me actually understanding that. > Because I think that what it can do is simply to do the ->poll() calls > outside the iocb locks, and then just attach the poll table to the > kioctx afterwards. We could do that on the submit side fairly easily. The problem is really the completion side, where I'd much avoid introducing a spurious context switch. Right now even with a NULL qproc we can't guarantee any of that. So we'll need to schedule out to a workqueue, and then from that schedule the potential multiple NULL qproc calls, which might actually block elsewhere even if __pollwait is never called. > This whole "poll must not block" is a complete red herring. It doesn't > come from any other requirements than BAD AIO GARBAGE CODE. I comes from the fact to avoid a totally pointless context switch. aio code itself works just fine called from a workqueue, we have exatly that case when file system do non-trivial operations in their end_io handler.
