Re: [PATCH -next 1/1] firmware: arm_scmi: Fix possible deadlock in shmem_tx_prepare()

[Sudeep Holla]

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.

-- 
Regards,
Sudeep