Messages in this thread | | | Date | Wed, 28 Jul 2021 09:31:25 +0100 | From | Cristian Marussi <> | Subject | Re: [PATCH v6 07/17] firmware: arm_scmi: Handle concurrent and out-of-order messages |
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On Thu, Jul 22, 2021 at 10:32:58AM +0200, Peter Hilber wrote: > On 19.07.21 11:14, Cristian Marussi wrote: > > On Thu, Jul 15, 2021 at 06:36:03PM +0200, Peter Hilber wrote: > > > On 12.07.21 16:18, Cristian Marussi wrote: > > [snip] > > > > > @@ -608,6 +755,7 @@ static int do_xfer(const struct scmi_protocol_handle *ph, > > > > xfer->hdr.protocol_id, xfer->hdr.seq, > > > > xfer->hdr.poll_completion); > > > > + xfer->state = SCMI_XFER_SENT_OK; > > > > > > To be completely safe, this assignment could also be protected by the > > > xfer->lock. > > > > > > > In fact this would be true being xfer->lock meant to protect the state but it > > seemed to me unnecessary here given that this is a brand new xfer with a > > brand new (monotonic) seq number so that any possibly late-received msg will > > carry an old stale seq number certainly different from this such that cannot be > > possibly mapped to this same xfer. (but just discarded on xfer lookup in > > xfer_command_acquire) > > > > The issue indeed could still exist only for do_xfer loops (as you pointed out > > already early on) where the seq_num is used, but in that case on a timeout we > > would have already bailed out of the loop and reported an error so any timed-out > > late received response would have been anyway discarded; so at the end I thought > > I could avoid spinlocking here. > > > > Thanks, > > Cristian > >
Hi Peter,
sorry for the late answer.
> > I mostly meant to refer to the possibility of a very fast response not > seeing this assignment, since the next line is > > > ret = info->desc->ops->send_message(cinfo, xfer); > > and during that a regular scmi_rx_callback(), reading xfer->state, can > already arrive. But maybe this is too theoretical. >
Right, that's a possibility indeed to account for even if remote: given that, though, no race is possible here on state as said, I'd still avoid the spinlock and related irq-off and opt instead for a barrier to avoid re-ordering and to be sure that the scmi_rx_callback() on the RX processor can see the latest value (a dmb(ish) + cache coherence magic should be enough)
Thanks, Cristian
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