Hi,
On 2019-10-25 11:30:35 -0600, Jens Axboe wrote:
> This is in preparation for adding opcodes that need to add new files
> in a process file table, system calls like open(2) or accept4(2).
>
> If an opcode needs this, it must set IO_WQ_WORK_NEEDS_FILES in the work
> item. If work that needs to get punted to async context have this
> set, the async worker will assume the original task file table before
> executing the work.
>
> Note that opcodes that need access to the current files of an
> application cannot be done through IORING_SETUP_SQPOLL.
Unfortunately this partially breaks sharing a uring across with forked
off processes, even though it initially appears to work:
> +static int io_uring_flush(struct file *file, void *data)
> +{
> + struct io_ring_ctx *ctx = file->private_data;
> +
> + io_uring_cancel_files(ctx, data);
> + if (fatal_signal_pending(current) || (current->flags & PF_EXITING))
> + io_wq_cancel_all(ctx->io_wq);
> + return 0;
> +}
Once one process having the uring fd open (even if it were just a fork
never touching the uring, I believe) exits, this prevents the uring from
being usable for any async tasks. The process exiting closes the fd,
which triggers flush. io_wq_cancel_all() sets IO_WQ_BIT_CANCEL, which
never gets unset, which causes all future async sqes to be be
immediately returned as -ECANCELLED by the worker, via io_req_cancelled.
It's not clear to me why a close() should cancel the the wq (nor clear
the entire backlog, after 1d7bb1d50fb4)? Couldn't that even just be a
dup()ed fd? Or a fork that immediately exec()s?
After rudely ifdefing out the above if, and reverting 44d282796f81, my
WIP io_uring using version of postgres appears to pass its tests - which
are very sparse at this point - again with 5.5-rc7.
Greetings,
Andres Freund
