Re: [PATCH V3 1/6] ublk: allow io buffer register/unregister command issued from other task contexts

[Ming Lei <ming.lei@redhat.com>]

On Mon, May 12, 2025 at 10:39:57AM -0700, Caleb Sander Mateos wrote:
> On Fri, May 9, 2025 at 8:06 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > `ublk_queue` is read only for io buffer register/unregister command. Both
> > `ublk_io` and block layer request are read-only for IO buffer register/
> > unregister command.
> >
> > So the two command can be issued from other task contexts.
> 
> I mentioned this before in
> https://lore.kernel.org/linux-block/CADUfDZqZ_9O7vUAYtxrrujWqPBuP05nBhCbzNuNsc9kJTmX2sA@mail.gmail.com/
> 
> But UBLK_IO_(UN)REGISTER_IO_BUF still reads io->flags. So it would be
> a race condition to handle it on a thread other than ubq_daemon, as
> ubq_daemon may concurrently modify io->flags. If you do want to
> support UBLK_IO_(UN)REGISTER_IO_BUF on other threads, the writes to
> io->flags should use WRITE_ONCE() and the reads on other threads
> should use READ_ONCE(). With those modifications, it should be safe
> because __ublk_check_and_get_req() atomically checks the state of the
> request and increments its reference count.

UBLK_IO_(UN)REGISTER_IO_BUF just reads the flag, if
UBLK_IO_FLAG_OWNED_BY_SRV is cleared, the OP is failed.

Otherwise, __ublk_check_and_get_req() covers everything because both
'ublk_io' and 'request' are pre-allocation. The only race is that new
recycled request buffer is registered, that is fine, because it can be
treated as logic bug.

So I think it isn't necessary to use READ_ONCE/WRITE_ONCE, or can you show
what the exact issue is?


Thanks, 
Ming