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

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

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:


 - 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.

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