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Date:   Sun, 20 Feb 2022 12:04:25 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Padmanabha Srinivasaiah <treasure4paddy@...il.com>
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 Sun, Feb 20, 2022 at 07:35:17PM +0100, Padmanabha Srinivasaiah wrote:
> 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.

Very good, thank you!

May I add your Tested-by?

							Thanx, Paul

> > 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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