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SubjectRe: [PATCH] rcu: wrong stall log when only one task blocked within RCU read-side
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On 5/16/21 2:36 AM, Paul E. McKenney wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On Sun, May 16, 2021 at 12:45:11AM +0800, yanfei.xu@windriver.com wrote:
>> From: Yanfei Xu <yanfei.xu@windriver.com>
>>
>> The for loop in rcu_print_task_stall() always omits the first blocked
>> task in RCU read-ride critical section. It will cause status of the
>> blocked task can't be printed and lead a wrong ndetected variable.
>> With the above, the wired stall log comes, All QSes seen, but qsmask
>> still is not zero:
>>
>> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
>> rcu: Tasks blocked on level-1 rcu_node (CPUs 12-23):
>> (detected by 15, t=6504 jiffies, g=164777, q=9011209)
>> rcu: All QSes seen, last rcu_preempt kthread activity 1
>> (4295252379-4295252378), jiffies_till_next_fqs=1, root ->qsmask 0x2
>> BUG: sleeping function called from invalid context at
>> include/linux/uaccess.h:156
>> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 70613, name:
>> msgstress04
>> INFO: lockdep is turned off.
>> Preemption disabled at:
>> [<ffff8000104031a4>] create_object.isra.0+0x204/0x4b0
>> CPU: 15 PID: 70613 Comm: msgstress04 Kdump: loaded Not tainted
>> 5.12.2-yoctodev-standard #1
>> Hardware name: Marvell OcteonTX CN96XX board (DT)
>> Call trace:
>> dump_backtrace+0x0/0x2cc
>> show_stack+0x24/0x30
>> dump_stack+0x110/0x188
>> ___might_sleep+0x214/0x2d0
>> __might_sleep+0x7c/0xe0
>>
>> Fixes: c583bcb8f5ed ("rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled")
>> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
>
> Apologies for that bug, and thank you for the fix! Queued for testing
> and review. I wordsmithed as usual, so could you please check to make
> sure that I did not mess anything up?
>

Ah, this new commit message is more clear. Thank you!

Regards,
Yanfei

> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 9855fdb508e2fa136551294861682d83b71daded
> Author: Yanfei Xu <yanfei.xu@windriver.com>
> Date: Sun May 16 00:45:11 2021 +0800
>
> rcu: Fix to include first blocked task in stall warning
>
> The for loop in rcu_print_task_stall() always omits ts[0], which points
> to the first task blocking the stalled grace period. This in turn fails
> to count this first task, which means that ndetected will be equal to
> zero when all CPUs have passed through their quiescent states and only
> one task is blocking the stalled grace period. This zero value for
> ndetected will in turn result in an incorrect "All QSes seen" message:
>
> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> rcu: Tasks blocked on level-1 rcu_node (CPUs 12-23):
> (detected by 15, t=6504 jiffies, g=164777, q=9011209)
> rcu: All QSes seen, last rcu_preempt kthread activity 1 (4295252379-4295252378), jiffies_till_next_fqs=1, root ->qsmask 0x2
> BUG: sleeping function called from invalid context at include/linux/uaccess.h:156
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 70613, name: msgstress04
> INFO: lockdep is turned off.
> Preemption disabled at:
> [<ffff8000104031a4>] create_object.isra.0+0x204/0x4b0
> CPU: 15 PID: 70613 Comm: msgstress04 Kdump: loaded Not tainted
> 5.12.2-yoctodev-standard #1
> Hardware name: Marvell OcteonTX CN96XX board (DT)
> Call trace:
> dump_backtrace+0x0/0x2cc
> show_stack+0x24/0x30
> dump_stack+0x110/0x188
> ___might_sleep+0x214/0x2d0
> __might_sleep+0x7c/0xe0
>
> This commit therefore fixes the loop to include ts[0].
>
> Fixes: c583bcb8f5ed ("rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled")
> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index f4152aa18f48..a10ea1f1f81f 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -280,8 +280,8 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
> break;
> }
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> - for (i--; i; i--) {
> - t = ts[i];
> + while (i) {
> + t = ts[--i];
> if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
> pr_cont(" P%d", t->pid);
> else
>

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