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