linux-kernel - Re: [PATCH] mailbox: fix completion order for blocking requests

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Message-ID: <20170411124538.GB4973@arm.com>
Date:   Tue, 11 Apr 2017 13:45:38 +0100
From:   Alexey Klimov <alexey.klimov@....com>
To:     Jassi Brar <jaswinder.singh@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mailbox: fix completion order for blocking requests

On Thu, Apr 06, 2017 at 10:45:26PM +0530, Jassi Brar wrote:
> On 6 April 2017 at 22:28, Alexey Klimov <alexey.klimov@....com> wrote:
> > Hi Jassi/Sudeep,
> >
> > On Wed, Mar 29, 2017 at 07:01:09PM +0100, Sudeep Holla wrote:
> >>
> >>
> >> On 29/03/17 18:43, Jassi Brar wrote:
> ...
> 
> >> > diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> >> > index 9dfbf7e..e06c50c 100644
> >> > --- a/drivers/mailbox/mailbox.c
> >> > +++ b/drivers/mailbox/mailbox.c
> >> > @@ -41,6 +41,7 @@ static int add_to_rbuf(struct mbox_chan *chan, void *mssg)
> >> >
> >> >     idx = chan->msg_free;
> >> >     chan->msg_data[idx] = mssg;
> >> > +   init_completion(&chan->tx_cmpl[idx]);
> >>
> >> reinit would be better.
> >
> Of course.
> 
> ....
> > From: Alexey Klimov <alexey.klimov@....com>
> > Date: Thu, 6 Apr 2017 13:57:02 +0100
> > Subject: [RFC][PATCH] mailbox: per-channel arrays with msg data and completion
> >  structures
> >
> > When a mailbox client doesn't serialize sending of the message itself,
> > and asks mailbox framework to block on mbox_send_message(), one
> > completion structure per channel is not enough. Client can make a few
> > mbox_send_message() calls at the same time, and there is no guaranteed
> > order of going to sleep on completion.
> >
> > If mailbox controller acks a message transfer, then tx_tick() wakes up
> > the first thread that waits on completion.
> > If mailbox controller doesn't ack the transfer and timeout happens, then
> > tx_tick() calls complete, and the next caller trying to sleep on
> > completion wakes up immediately.
> >
> > This patch fixes this by changing completion structures to be inserted
> > into an array that contains a) pointer to data provided by client and
> > b) the completion structure. Thus active_req field tracks the index of
> > the current running request that was submitted to mailbox controller.
> >
> > Signed-off-by: Alexey Klimov <alexey.klimov@....com>
> > ---
> >  drivers/mailbox/mailbox.c          | 40 +++++++++++++++++++++++---------------
> >  drivers/mailbox/pcc.c              | 10 +++++++---
> >  include/linux/mailbox_controller.h | 24 +++++++++++++++++------
> ...
> >  3 files changed, 49 insertions(+), 25 deletions(-)
> >
>  Versus   4 files changed, 17 insertions(+), 8 deletions(-)
> 
> I think we should just keep it simpler if it works just as fine.

Along with this patch you still need at least one patch from Sudeep with subject:
"[PATCH 1/3] mailbox: always wait in mbox_send_message for blocking Tx mode"

Decision to block or not shouldn't depend on racy reading of active_req field.

Best regards,
Alexey Klimov.