Re: [PATCH v2 03/14] io_uring/cmd: make io_uring_cmd_done irq safe

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

On Mon, Mar 18, 2024 at 12:46:25PM +0000, Pavel Begunkov wrote:
> On 3/18/24 11:59, Ming Lei wrote:
> > On Mon, Mar 18, 2024 at 11:50:32AM +0000, Pavel Begunkov wrote:
> > > On 3/18/24 08:10, Ming Lei wrote:
> > > > On Mon, Mar 18, 2024 at 12:41:48AM +0000, Pavel Begunkov wrote:
> > > > > io_uring_cmd_done() is called from the irq context and is considered to
> > > > > be irq safe, however that's not the case if the driver requires
> > > > > cancellations because io_uring_cmd_del_cancelable() could try to take
> > > > > the uring_lock mutex.
> > > > > 
> > > > > Clean up the confusion, by deferring cancellation handling to
> > > > > locked task_work if we came into io_uring_cmd_done() from iowq
> > > > > or other IO_URING_F_UNLOCKED path.
> > > > > 
> > > > > Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> > > > > ---
> > > > >    io_uring/uring_cmd.c | 24 +++++++++++++++++-------
> > > > >    1 file changed, 17 insertions(+), 7 deletions(-)
> > > > > 
> > > > > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> > > > > index ec38a8d4836d..9590081feb2d 100644
> > > > > --- a/io_uring/uring_cmd.c
> > > > > +++ b/io_uring/uring_cmd.c
> > > > > @@ -14,19 +14,18 @@
> > > > >    #include "rsrc.h"on   #include "uring_cmd.h"
> > > > > -static void io_uring_cmd_del_cancelable(struct io_uring_cmd *cmd,
> > > > > -		unsigned int issue_flags)
> > > > > +static void io_uring_cmd_del_cancelable(struct io_uring_cmd *cmd)
> > > > >    {
> > > > >    	struct io_kiocb *req = cmd_to_io_kiocb(cmd);
> > > > >    	struct io_ring_ctx *ctx = req->ctx;
> > > > > +	lockdep_assert_held(&ctx->uring_lock);
> > > > > +
> > > > >    	if (!(cmd->flags & IORING_URING_CMD_CANCELABLE))
> > > > >    		return;
> > > > >    	cmd->flags &= ~IORING_URING_CMD_CANCELABLE;
> > > > > -	io_ring_submit_lock(ctx, issue_flags);
> > > > >    	hlist_del(&req->hash_node);
> > > > > -	io_ring_submit_unlock(ctx, issue_flags);
> > > > >    }
> > > > >    /*
> > > > > @@ -44,6 +43,9 @@ void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
> > > > >    	struct io_kiocb *req = cmd_to_io_kiocb(cmd);
> > > > >    	struct io_ring_ctx *ctx = req->ctx;
> > > > > +	if (WARN_ON_ONCE(ctx->flags & IORING_SETUP_IOPOLL))
> > > > > +		return;
> > > > > +
> > > > 
> > > > This way limits cancelable command can't be used in iopoll context, and
> > > > it isn't reasonable, and supporting iopoll has been in ublk todo list,
> > > > especially single ring context is shared for both command and normal IO.
> > > 
> > > That's something that can be solved when it's needed, and hence the
> > > warning so it's not missed. That would need del_cancelable on the
> > > ->iopoll side, but depends on the "leaving in cancel queue"
> > > problem resolution.
> > 
> > The current code is actually working with iopoll, so adding the warning
> > can be one regression. Maybe someone has been using ublk with iopoll.
> 
> Hmm, I don't see ->uring_cmd_iopoll implemented anywhere, and w/o
> it io_uring should fail the request:
> 
> int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
> {
> 	...
> 	if (ctx->flags & IORING_SETUP_IOPOLL) {
> 		if (!file->f_op->uring_cmd_iopoll)
> 			return -EOPNOTSUPP;
> 		issue_flags |= IO_URING_F_IOPOLL;
> 	}
> }
> 
> Did I miss it somewhere?

Looks I missed the point, now the WARN() is fine.


Thanks,
Ming