Re: [PATCH] rcu-tasks: Fix race in schedule and flush work

From: Paul E. McKenney
Date: Sun Feb 20 2022 - 15:04:33 EST

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@xxxxxxxxx/T/
> > > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > 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@xxxxxxxxx/T/
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >
> > 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);
> > }