Re: [PATCH] mailbox: cancel timer before starting it

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi Jerome,

On Thursday 15 Oct 2020 at 15:58:30 (+0200), Jerome Brunet wrote:
> 
> On Thu 15 Oct 2020 at 15:46, Ionela Voinescu <ionela.voinescu@arm.com> wrote:
> 
> > Hi guys,
> >
> > On Wednesday 23 Sep 2020 at 14:39:16 (+0200), Jerome Brunet wrote:
> >> If the txdone is done by polling, it is possible for msg_submit() to start
> >> the timer while txdone_hrtimer() callback is running. If the timer needs
> >> recheduling, it could already be enqueued by the time hrtimer_forward_now()
> >> is called, leading hrtimer to loudly complain.
> >> 
> >> WARNING: CPU: 3 PID: 74 at kernel/time/hrtimer.c:932 hrtimer_forward+0xc4/0x110
> >> CPU: 3 PID: 74 Comm: kworker/u8:1 Not tainted 5.9.0-rc2-00236-gd3520067d01c-dirty #5
> >> Hardware name: Libre Computer AML-S805X-AC (DT)
> >> Workqueue: events_freezable_power_ thermal_zone_device_check
> >> pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
> >> pc : hrtimer_forward+0xc4/0x110
> >> lr : txdone_hrtimer+0xf8/0x118
> >> [...]
> >> 
> >> Canceling the timer before starting it ensure that the timer callback is
> >> not running when the timer is started, solving this race condition.
> >> 
> >> Fixes: 0cc67945ea59 ("mailbox: switch to hrtimer for tx_complete polling")
> >> Reported-by: Da Xue <da@libre.computer>
> >> Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
> >> ---
> >>  drivers/mailbox/mailbox.c | 8 ++++++--
> >>  1 file changed, 6 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> >> index 0b821a5b2db8..34f9ab01caef 100644
> >> --- a/drivers/mailbox/mailbox.c
> >> +++ b/drivers/mailbox/mailbox.c
> >> @@ -82,9 +82,13 @@ static void msg_submit(struct mbox_chan *chan)
> >>  exit:
> >>  	spin_unlock_irqrestore(&chan->lock, flags);
> >>  
> >> -	if (!err && (chan->txdone_method & TXDONE_BY_POLL))
> >> -		/* kick start the timer immediately to avoid delays */
> >> +	if (!err && (chan->txdone_method & TXDONE_BY_POLL)) {
> >> +		/* Disable the timer if already active ... */
> >> +		hrtimer_cancel(&chan->mbox->poll_hrt);
> >> +
> >> +		/* ... and kick start it immediately to avoid delays */
> >>  		hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
> >> +	}
> >>  }
> >>  
> >>  static void tx_tick(struct mbox_chan *chan, int r)
> >
> > I've tracked a regression back to this commit. Details to reproduce:
> 
> Hi Ionela,
> 
> I don't have access to your platform and I don't get what is going on
> from the log below.
> 
> Could you please give us a bit more details about what is going on ?
> 

I'm not familiar with the mailbox subsystem, so the best I can do right
now is to add Sudeep to Cc, in case this conflicts in some way with the
ARM MHU patches [1].

In the meantime I'll get some traces and get more familiar with the
code.

[1]
https://lore.kernel.org/linux-arm-kernel/20201009113129.uqw5zrqztjgw6vga@bogus/

Hope it helps,
Ionela.

> All this patch does is add hrtimer_cancel().
> * It is needed if the timer had already been started, which is
>   appropriate AFAIU
> * It is a NO-OP is the timer is not active.
> 
> >
> >
> >  - HEAD: (linux-next)
> >    * 62c04453381e  Jerome Brunet   3 weeks ago mailbox: cancel timer before starting it
> >
> >  - Platform: arm64 Juno R0 and Juno R2 [1]
> >
> >  - Partial log:
> > 	[    0.000000] Booting Linux on physical CPU 0x0000000100 [0x410fd030]
> > 	[    0.000000] Linux version 5.9.0-rc8-01722-g62c04453381e () (aarch64-none-linux-gnu-gcc (GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10)) 9.2.1 20191025, GNU ld (GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10)) 2.33.1.20191209) #175 SMP PREEMPT Thu Oct 15 14:17:41 BST 2020
> > 	[    0.000000] Machine model: ARM Juno development board (r0)
> > 	[..]
> > 	[    1.714340] mhu 2b1f0000.mhu: ARM MHU Mailbox registered
> > 	[    1.722768] NET: Registered protocol family 17
> > 	[    1.727364] 9pnet: Installing 9P2000 support
> > 	[    1.731689] Key type dns_resolver registered
> > 	[    1.735474] usb 1-1: new high-speed USB device number 2 using ehci-platform
> > 	[    1.736407] registered taskstats version 1
> > 	[    1.747061] Loading compiled-in X.509 certificates
> > 	[    1.755885] scpi_protocol scpi: SCP Protocol 1.2 Firmware 1.21.0 version
> > 	[    1.770484] cpu cpu0: EM: created perf domain
> > 	[    1.778505] cpu cpu1: EM: created perf domain
> > 	[    1.807449] scpi_clocks scpi:clocks: failed to register clock 'pxlclk'
> > 	[    1.897593] hub 1-1:1.0: USB hub found
> > 	[    1.901656] hub 1-1:1.0: 4 ports detected
> > 	[    2.559453] atkbd serio0: keyboard reset failed on 1c060000.kmi
> > 	[   22.787431] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > 	[   22.793536] rcu:     1-...0: (1 ticks this GP) idle=222/1/0x4000000000000002 softirq=63/64 fqs=2626
> > 	[   22.802421]  (detected by 2, t=5255 jiffies, g=-991, q=9)
> > 	[   22.807823] Task dump for CPU 1:
> > 	[   22.811049] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[   22.820980] Call trace:
> > 	[   22.823429]  __switch_to+0x138/0x198
> > 	[   23.583444] rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 1-... } 5443 jiffies s: 49 root: 0x2/.
> > 	[   23.593995] rcu: blocking rcu_node structures:
> > 	[   23.598449] Task dump for CPU 1:
> > 	[   23.601680] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[   23.611619] Call trace:
> > 	[   23.614064]  __switch_to+0x138/0x198
> > 	[   85.807430] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > 	[   85.813534] rcu:     1-...0: (1 ticks this GP) idle=222/1/0x4000000000000002 softirq=63/64 fqs=10502
> > 	[   85.822506]  (detected by 2, t=21009 jiffies, g=-991, q=9)
> > 	[   85.827994] Task dump for CPU 1:
> > 	[   85.831220] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[   85.841150] Call trace:
> > 	[   85.843596]  __switch_to+0x138/0x198
> > 	[   87.071446] rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 1-... } 21315 jiffies s: 49 root: 0x2/.
> > 	[   87.082088] rcu: blocking rcu_node structures:
> > 	[   87.086540] Task dump for CPU 1:
> > 	[   87.089773] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[   87.099708] Call trace:
> > 	[   87.102155]  __switch_to+0x138/0x198
> > 	[  148.827442] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > 	[  148.833565] rcu:     1-...0: (1 ticks this GP) idle=222/1/0x4000000000000002 softirq=63/64 fqs=18377
> > 	[  148.842543]  (detected by 4, t=36762 jiffies, g=-991, q=9)
> > 	[  148.848037] Task dump for CPU 1:
> > 	[  148.851268] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[  148.861207] Call trace:
> > 	[  148.863663]  __switch_to+0x138/0x198
> > 	[  150.559443] rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 1-... } 37187 jiffies s: 49 root: 0x2/.
> > 	[  150.570082] rcu: blocking rcu_node structures:
> > 	[  150.574535] Task dump for CPU 1:
> > 	[  150.577767] task:swapper/1       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x0000002a
> > 	[  150.587705] Call trace:
> > 	[  150.590151]  __switch_to+0x138/0x198
> >
> >  - Commit working as expected:
> >    * 558e4c36ec9f  Krzysztof Kozlowski     7 weeks ago     maiblox: mediatek: Fix handling of platform_get_irq() error
> >
> >
> > [1] https://developer.arm.com/tools-and-software/development-boards/juno-development-board
> >
> > Thank you,
> > Ionela.
> >
> >> -- 
> >> 2.25.4
> >> 
> >> 
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