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Date:   Fri, 14 Oct 2022 13:23:03 +0100
From:   Cristian Marussi <cristian.marussi@arm.com>
To:     Sudeep Holla <sudeep.holla@arm.com>
Cc:     YaxiongTian <iambestgod@outlook.com>, iambestgod@qq.com,
        james.quinlan@broadcom.com, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        tianyaxiong@kylinos.cn
Subject: Re: [PATCH -next 1/1] firmware: arm_scmi: Fix possible deadlock in
 shmem_tx_prepare()
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On Fri, Oct 14, 2022 at 12:56:39PM +0100, Sudeep Holla wrote:
> On Thu, Oct 13, 2022 at 05:02:15PM +0100, Cristian Marussi wrote:
> > On Thu, Oct 13, 2022 at 03:05:43PM +0800, YaxiongTian wrote:
> > > Hi Cristian
> > > 
> > > �� There may be a problem with my qq email client, � I don't see my mail in
> > > the
> > > 
> > > communityI had to switch outlook email.Forgive me if you've received
> > > multiple emails.
> > > 
> > No worries.
> > 
> > > >Problem is anyway, as you said, you'll have to pick this timeout from the
> > > >related transport scmi_desc (even if as of now the max_rx_timeout for
> > > >all existent shared mem transport is the same..) and this means anyway
> > > >adding more complexity to the chain of calls to just to print a warn of
> > > >some kind in a rare error-situation from which you cannot recover anyway.
> > > 
> > > � Yes,it has add more complexity about Monitorring this time.For system
> > > stability,the safest thing to do is to abort the transmission.But this will
> > > lose performance due to more complexity in such unusual situation.
> > > 
> > > >Due to other unrelated discussions, I was starting to think about
> > > >exposing some debug-only (Kconfig dependent) SCMI stats like timeouts,
> > > errors,
> > > >unpexpected/OoO/late_replies in order to ease the debug and monitoring
> > > >of the health of a running SCMI stack: maybe this could be a place where
> > > >to flag this FW issues without changing the spinloop above (or
> > > >to add the kind of timeout you mentioned but only when some sort of
> > > >CONFIG_SCMI_DEBUG is enabled...)...still to fully think it through, though.
> > > 
> > > � I think it should active report warn or err rather than user queries the
> > > information manually.(i.e fs_debug way).Becasue in system startup\S1\S3\S4,
> > > user can not queries this flag in Fw,they need get stuck message
> > > immediately.
> > > 
> > 
> > Looking more closely at this, I experimented a bit with an SCMI stack based on
> > mailbox transport in which I had forcefully set the spin_until_cond() to
> > spin forever.
> > 
> > Even though on a normal SCMI system when the SCMI stack fails at boot
> > the system is supposed to boot anyway (maybe slower), this particular
> > failure in TX path led indeed to a system that does not boot at all and
> > spits out an infinite sequence of:
> > 
> > [ 2924.499486] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [ 2924.505596] rcu:     2-...0: (0 ticks this GP) idle=1be4/1/0x4000000000000000 softirq=50/50 fqs=364757
> > [ 2924.514672]  (detected by 4, t=730678 jiffies, g=-1119, q=134 ncpus=6)
> > [ 2924.521215] Task dump for CPU 2:
> > [ 2924.524445] task:kworker/u12:0   state:R  running task     stack:    0 pid:    9 ppid:     2 flags:0x0000000a
> > [ 2924.534391] Workqueue: events_unbound deferred_probe_work_func
> > [ 2924.540244] Call trace:
> > [ 2924.542691]  __switch_to+0xe4/0x1b8
> > [ 2924.546189]  deferred_probe_work_func+0xa4/0xf8
> > [ 2924.550731]  process_one_work+0x208/0x480
> > [ 2924.554754]  worker_thread+0x230/0x428
> > [ 2924.558514]  kthread+0x114/0x120
> > [ 2924.561752]  ret_from_fork+0x10/0x20
> > 
> > I imagine this is the annoying thing you want to avoid.
> > 
> > So experimenting a bit with a patch similar to yours (ignoring the timeout
> > config issues and using the static cnt to temporarily stuck and revive the SCMI
> > transport)
> > 
> > ------>8-----
> > diff --git a/drivers/firmware/arm_scmi/shmem.c b/drivers/firmware/arm_scmi/shmem.c
> > index 0e3eaea5d852..6dde669abd03 100644
> > --- a/drivers/firmware/arm_scmi/shmem.c
> > +++ b/drivers/firmware/arm_scmi/shmem.c
> > @@ -8,6 +8,7 @@
> >  #include <linux/io.h>
> >  #include <linux/processor.h>
> >  #include <linux/types.h>
> >  
> >  #include "common.h"
> >  
> > @@ -29,17 +30,28 @@ struct scmi_shared_mem {
> >         u8 msg_payload[];
> >  };
> >  
> > +static int cnt = 50;
> >  void shmem_tx_prepare(struct scmi_shared_mem __iomem *shmem,
> >                       struct scmi_xfer *xfer)
> >  {
> > +       ktime_t stop;
> > +
> >         /*
> >          * Ideally channel must be free by now unless OS timeout last
> >          * request and platform continued to process the same, wait
> >          * until it releases the shared memory, otherwise we may endup
> >          * overwriting its response with new message payload or vice-versa
> >          */
> > -       spin_until_cond(ioread32(&shmem->channel_status) &
> > -                       SCMI_SHMEM_CHAN_STAT_CHANNEL_FREE);
> > +       stop = ktime_add_ms(ktime_get(), 35);
> > +       spin_until_cond(((--cnt > 0) && ioread32(&shmem->channel_status) &
> > +                       SCMI_SHMEM_CHAN_STAT_CHANNEL_FREE) ||
> > +                       ktime_after(ktime_get(), stop));
> > +       if (ktime_after(ktime_get(), stop)) {
> > +               pr_warn_once("TX Timeout !\n");
> > +               cnt = 10;
> > +               return;
> > +       }
> > +
> >         /* Mark channel busy + clear error */
> >         iowrite32(0x0, &shmem->channel_status);
> >         iowrite32(xfer->hdr.poll_completion ? 0 : SCMI_SHMEM_FLAG_INTR_ENABLED,
> > ----8<-------------
> > 
> > With the above I had in fact a system that could boot even with a
> > failing/stuck SCMI transport, but, as expected the SCMI stack
> > functionality was totally compromised after the first timeout with no
> > possibility to recover.
> > 
> > Moreover I was thinking at what could happen if later on after boot the
> > SCMI server should end in some funny/hogged condition so that it is,
> > only temporarily, a bit slower to answer and release the channel: with
> > the current implemenation the Kernel agent will spin just a little bit
> > more waiting for the channel to be freed and then everything carries
> > without much hassle, while with this possible new timing-out solution
> > we could end up dropping that transmission and compromising the whole
> > transport fucntionality for all the subsequent transmissions.
> > 
> > So, again, I'm not sure it is worth making such a change even for debug
> > purposes, given that in the worst scenario above you end up with a
> > system stuck at boot but for which the SCMI stack is anyway compromised
> > and where the only solution is fixing the server FW really.
> > 
> > I'll ask Sudeep is thoughts about the possible hang.
> >
> 
> I am fine with the patch as it provides more info on what is going wrong
> in the system. Please post the patch separately with all the info/background.
> 

Ok, I'll cleanup and post adding Reported/Suggested-by: YaxiongTian

I'm inclined to set the timeout comfortably more than the transport RX
timeout. (2xrx_timeout ?) to account for overhead and avoiding to bail
out on some transient delays.

Thanks,
Cristian