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Date:   Sun, 20 Feb 2022 19:35:17 +0100
From:   Padmanabha Srinivasaiah <treasure4paddy@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] rcu-tasks: Fix race in schedule and flush work

On Thu, Feb 17, 2022 at 09:43:30AM -0800, Paul E. McKenney wrote:
> On Thu, Feb 17, 2022 at 04:25:19PM +0100, Padmanabha Srinivasaiah wrote:
> > While booting secondary CPUs, cpus_read_[lock/unlock] not keeping online
> > cpumask stable. The transient online mask results in below calltrace.
> > 
> > [    0.324121] CPU1: Booted secondary processor 0x0000000001 [0x410fd083]
> > [    0.346652] Detected PIPT I-cache on CPU2
> > [    0.347212] CPU2: Booted secondary processor 0x0000000002 [0x410fd083]
> > [    0.377255] Detected PIPT I-cache on CPU3
> > [    0.377823] CPU3: Booted secondary processor 0x0000000003 [0x410fd083]
> > [    0.379040] ------------[ cut here ]------------
> > [    0.383662] WARNING: CPU: 0 PID: 10 at kernel/workqueue.c:3084 __flush_work+0x12c/0x138
> > [    0.384850] Modules linked in:
> > [    0.385403] CPU: 0 PID: 10 Comm: rcu_tasks_rude_ Not tainted 5.17.0-rc3-v8+ #13
> > [    0.386473] Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT)
> > [    0.387289] pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [    0.388308] pc : __flush_work+0x12c/0x138
> > [    0.388970] lr : __flush_work+0x80/0x138
> > [    0.389620] sp : ffffffc00aaf3c60
> > [    0.390139] x29: ffffffc00aaf3d20 x28: ffffffc009c16af0 x27: ffffff80f761df48
> > [    0.391316] x26: 0000000000000004 x25: 0000000000000003 x24: 0000000000000100
> > [    0.392493] x23: ffffffffffffffff x22: ffffffc009c16b10 x21: ffffffc009c16b28
> > [    0.393668] x20: ffffffc009e53861 x19: ffffff80f77fbf40 x18: 00000000d744fcc9
> > [    0.394842] x17: 000000000000000b x16: 00000000000001c2 x15: ffffffc009e57550
> > [    0.396016] x14: 0000000000000000 x13: ffffffffffffffff x12: 0000000100000000
> > [    0.397190] x11: 0000000000000462 x10: ffffff8040258008 x9 : 0000000100000000
> > [    0.398364] x8 : 0000000000000000 x7 : ffffffc0093c8bf4 x6 : 0000000000000000
> > [    0.399538] x5 : 0000000000000000 x4 : ffffffc00a976e40 x3 : ffffffc00810444c
> > [    0.400711] x2 : 0000000000000004 x1 : 0000000000000000 x0 : 0000000000000000
> > [    0.401886] Call trace:
> > [    0.402309]  __flush_work+0x12c/0x138
> > [    0.402941]  schedule_on_each_cpu+0x228/0x278
> > [    0.403693]  rcu_tasks_rude_wait_gp+0x130/0x144
> > [    0.404502]  rcu_tasks_kthread+0x220/0x254
> > [    0.405264]  kthread+0x174/0x1ac
> > [    0.405837]  ret_from_fork+0x10/0x20
> > [    0.406456] irq event stamp: 102
> > [    0.406966] hardirqs last  enabled at (101): [<ffffffc0093c8468>] _raw_spin_unlock_irq+0x78/0xb4
> > [    0.408304] hardirqs last disabled at (102): [<ffffffc0093b8270>] el1_dbg+0x24/0x5c
> > [    0.409410] softirqs last  enabled at (54): [<ffffffc0081b80c8>] local_bh_enable+0xc/0x2c
> > [    0.410645] softirqs last disabled at (50): [<ffffffc0081b809c>] local_bh_disable+0xc/0x2c
> > [    0.411890] ---[ end trace 0000000000000000 ]---
> > [    0.413000] smp: Brought up 1 node, 4 CPUs
> > [    0.413762] SMP: Total of 4 processors activated.
> > [    0.414566] CPU features: detected: 32-bit EL0 Support
> > [    0.415414] CPU features: detected: 32-bit EL1 Support
> > [    0.416278] CPU features: detected: CRC32 instructions
> > [    0.447021] Callback from call_rcu_tasks_rude() invoked.
> > [    0.506693] Callback from call_rcu_tasks() invoked.
> > 
> > Link: https://lore.kernel.org/lkml/20220210184319.25009-1-treasure4paddy@gmail.com/T/
> > Suggested-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@...il.com>
> 
> Thank you, Padmanabha!  I have queued this with additional explanation
> in the commit log as shown below.

Thanks paul.
> 
> If you have not already tested this by repeatedly rebooting, for
> example continuously in an overnight test, could you please do so?
> (I am assuming that your system can do an automated reboot loop, so no,
> I am not asking you to stay up all night!)
> 
Yes, tested and issue doesn't appear.

> Should this somehow recur with larger systems, in which the call starts
> with (say) five CPUs online but has trouble with the sixth CPU, there
> are several ways to address it:
> 
> 1.	Make cpus_read_lock() act as expected.
>
> 2.	Make the first for_each_online_cpu() in schedule_on_each_cpu()
> 	instead be for_each_possible_cpu(), always do the INIT_WORK(),
> 	but invoke schedule_work_on() only if cpu_online(cpu).
>
Yes, agree and was also suggested by Tejun Heo. Will also submit same, in
other thread.

> 3.	Other ideas here, when and if.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit f6823834aa2e2e581ca627238fa3ad3e0a727c08
> Author: Padmanabha Srinivasaiah <treasure4paddy@...il.com>
> Date:   Thu Feb 17 16:25:19 2022 +0100
> 
>     rcu-tasks: Fix race in schedule and flush work
>     
>     While booting secondary CPUs, cpus_read_[lock/unlock] is not keeping
>     online cpumask stable. The transient online mask results in below
>     calltrace.
>     
>     [    0.324121] CPU1: Booted secondary processor 0x0000000001 [0x410fd083]
>     [    0.346652] Detected PIPT I-cache on CPU2
>     [    0.347212] CPU2: Booted secondary processor 0x0000000002 [0x410fd083]
>     [    0.377255] Detected PIPT I-cache on CPU3
>     [    0.377823] CPU3: Booted secondary processor 0x0000000003 [0x410fd083]
>     [    0.379040] ------------[ cut here ]------------
>     [    0.383662] WARNING: CPU: 0 PID: 10 at kernel/workqueue.c:3084 __flush_work+0x12c/0x138
>     [    0.384850] Modules linked in:
>     [    0.385403] CPU: 0 PID: 10 Comm: rcu_tasks_rude_ Not tainted 5.17.0-rc3-v8+ #13
>     [    0.386473] Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT)
>     [    0.387289] pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>     [    0.388308] pc : __flush_work+0x12c/0x138
>     [    0.388970] lr : __flush_work+0x80/0x138
>     [    0.389620] sp : ffffffc00aaf3c60
>     [    0.390139] x29: ffffffc00aaf3d20 x28: ffffffc009c16af0 x27: ffffff80f761df48
>     [    0.391316] x26: 0000000000000004 x25: 0000000000000003 x24: 0000000000000100
>     [    0.392493] x23: ffffffffffffffff x22: ffffffc009c16b10 x21: ffffffc009c16b28
>     [    0.393668] x20: ffffffc009e53861 x19: ffffff80f77fbf40 x18: 00000000d744fcc9
>     [    0.394842] x17: 000000000000000b x16: 00000000000001c2 x15: ffffffc009e57550
>     [    0.396016] x14: 0000000000000000 x13: ffffffffffffffff x12: 0000000100000000
>     [    0.397190] x11: 0000000000000462 x10: ffffff8040258008 x9 : 0000000100000000
>     [    0.398364] x8 : 0000000000000000 x7 : ffffffc0093c8bf4 x6 : 0000000000000000
>     [    0.399538] x5 : 0000000000000000 x4 : ffffffc00a976e40 x3 : ffffffc00810444c
>     [    0.400711] x2 : 0000000000000004 x1 : 0000000000000000 x0 : 0000000000000000
>     [    0.401886] Call trace:
>     [    0.402309]  __flush_work+0x12c/0x138
>     [    0.402941]  schedule_on_each_cpu+0x228/0x278
>     [    0.403693]  rcu_tasks_rude_wait_gp+0x130/0x144
>     [    0.404502]  rcu_tasks_kthread+0x220/0x254
>     [    0.405264]  kthread+0x174/0x1ac
>     [    0.405837]  ret_from_fork+0x10/0x20
>     [    0.406456] irq event stamp: 102
>     [    0.406966] hardirqs last  enabled at (101): [<ffffffc0093c8468>] _raw_spin_unlock_irq+0x78/0xb4
>     [    0.408304] hardirqs last disabled at (102): [<ffffffc0093b8270>] el1_dbg+0x24/0x5c
>     [    0.409410] softirqs last  enabled at (54): [<ffffffc0081b80c8>] local_bh_enable+0xc/0x2c
>     [    0.410645] softirqs last disabled at (50): [<ffffffc0081b809c>] local_bh_disable+0xc/0x2c
>     [    0.411890] ---[ end trace 0000000000000000 ]---
>     [    0.413000] smp: Brought up 1 node, 4 CPUs
>     [    0.413762] SMP: Total of 4 processors activated.
>     [    0.414566] CPU features: detected: 32-bit EL0 Support
>     [    0.415414] CPU features: detected: 32-bit EL1 Support
>     [    0.416278] CPU features: detected: CRC32 instructions
>     [    0.447021] Callback from call_rcu_tasks_rude() invoked.
>     [    0.506693] Callback from call_rcu_tasks() invoked.
>     
>     This commit therefore fixes this issue by applying a single-CPU
>     optimization to the RCU Tasks Rude grace-period process.  The key point
>     here is that the purpose of this RCU flavor is to force a schedule on
>     each online CPU since some past event.  But the rcu_tasks_rude_wait_gp()
>     function runs in the context of the RCU Tasks Rude's grace-period kthread,
>     so there must already have been a context switch on the current CPU since
>     the call to either synchronize_rcu_tasks_rude() or call_rcu_tasks_rude().
>     So if there is only a single CPU online, RCU Tasks Rude's grace-period
>     kthread does not need to anything at all.
>     
>     It turns out that the rcu_tasks_rude_wait_gp() function's call to
>     schedule_on_each_cpu() causes problems during early boot.  During that
>     time, there is only one online CPU, namely the boot CPU.  Therefore,
>     applying this single-CPU optimization fixes early-boot instances of
>     this problem.
>     
>     Link: https://lore.kernel.org/lkml/20220210184319.25009-1-treasure4paddy@gmail.com/T/
>     Suggested-by: Paul E. McKenney <paulmck@...nel.org>
>     Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@...il.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index d73e32d803438..f7fdb349f8b8e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -955,6 +955,9 @@ static void rcu_tasks_be_rude(struct work_struct *work)
>  // Wait for one rude RCU-tasks grace period.
>  static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>  {
> +	if (num_online_cpus() <= 1)
> +		return;	// Fastpath for only one CPU.
> +
>  	rtp->n_ipis += cpumask_weight(cpu_online_mask);
>  	schedule_on_each_cpu(rcu_tasks_be_rude);
>  }

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