Re: [PATCH v2] rcu: Use an intermediate irq_work to start process_srcu()

[Boqun Feng <boqun@kernel.org>]

On Sat, Jun 20, 2026 at 05:25:25AM -0400, Emil Renner Berthing wrote:
> Quoting Boqun Feng (2026-06-18 17:30:10)
> > On Thu, Jun 18, 2026 at 08:11:36AM -0500, Emil Renner Berthing wrote:
> > > Quoting Boqun Feng (2026-03-20 23:29:16)
> > > > Since commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms
> > > > of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
> > > > happen basically everywhere (including where a scheduler lock is held),
> > > > call_srcu() now needs to avoid acquiring scheduler lock because
> > > > otherwise it could cause deadlock [1]. Fix this by following what the
> > > > previous RCU Tasks Trace did: using an irq_work to delay the queuing of
> > > > the work to start process_srcu().
> > > >
> > > > [boqun: Apply Joel's feedback]
> > > > [boqun: Apply Andrea's test feedback]
> > > >
> > > > Reported-by: Andrea Righi <arighi@nvidia.com>
> > > > Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/
> > > > Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
> > > > Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1]
> > > > Suggested-by: Zqiang <qiang.zhang@linux.dev>
> > > > Tested-by: Andrea Righi <arighi@nvidia.com>
> > > > Signed-off-by: Boqun Feng <boqun@kernel.org>
> > >
> > > Thanks for the patch, which is now upstream as
> > > 7c405fb3279b ("rcu: Use an intermediate irq_work to start process_srcu()")
> > >
> > > Unfortunately it seems to halt booting on the Allwinner D1 single-core RISC-V
> > > SoC both on the 7.0 and 7.1 kernels:
> > >
> >
> > By "single-core", is it only one CPU in that system?
> >
> > > https://esmil.dk/d1-7.0.txt
> > > https://esmil.dk/d1-7.1.txt
> > >
> 
> Yes, the SoC only has one CPU core capable of running Linux at least.
> 
> > From the callstack,
> >
> > [  243.002818] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  243.010681] task:kworker/0:0     state:D stack:0     pid:9     tgid:9     ppid:2      task_flags:0x4208060 flags:0x00000000
> > [  243.021858] Workqueue: device_link_wq device_link_release_fn
> > [  243.027587] Call Trace:
> > [  243.030079] [<ffffffff80745b66>] __schedule+0x232/0x530
> > [  243.035365] [<ffffffff80745e82>] schedule+0x1e/0x94
> > [  243.040298] [<ffffffff8074aa7a>] schedule_timeout+0x6e/0xa8
> > [  243.045929] [<ffffffff8004d6a0>] wakeup_preempt+0x8c/0x98
> > [  243.051380] [<ffffffff80746aee>] wait_for_completion+0x3a/0xb0
> > [  243.057267] [<ffffffff80087226>] __synchronize_srcu.part.0+0x4e/0x60
> > [  243.063712] [<ffffffff80084414>] rcu_tasks_get_gp_data+0xc/0x10
> > [  243.069699] [<ffffffff80490898>] device_link_release_fn+0x14/0x80
> > [  243.075850] [<ffffffff8003dbd0>] process_one_work+0xf8/0x1c8
> > [  243.081583] [<ffffffff8003e282>] worker_thread+0x11a/0x254
> > [  243.087127] [<ffffffff8003e164>] rescuer_thread+0x3a0/0x3a4
> > [  243.092755] [<ffffffff8003e164>] rescuer_thread+0x3a0/0x3a4
> > [  243.098381] [<ffffffff8004499e>] kthread+0xbe/0xe4
> > [  243.103231] [<ffffffff8001349e>] ret_from_fork_kernel+0x6/0xd0
> > [  243.109127] [<ffffffff800517a8>] schedule_tail+0x8/0xac
> > [  243.114410] [<ffffffff800448dc>] kthreads_online_cpu+0x0/0x4
> > [  243.120121] [<ffffffff8074be0a>] ret_from_fork_kernel_asm+0x12/0x18
> >
> > I will guess for some reasons the irq_work was missed or the IPI never
> > happened. Is it possible for you to enable trace points ipi_send_cpu and
> > ipi_entry to get more information?
> 
> I have to admit I've not used tracepoints before, but it seems like only
> ipi_send_cpu and ipi_send_cpumask is available on RISC-V.
> I've now added trace_event=ipi:* and tp_printk to the command line, and this

Thank you! Yeah, this is what I meant ;-)

> is what I get:
> https://esmil.dk/d1-7.0-2.txt
> 


[    0.071132] rcu: 	Max phase no-delay instances is 1000.
[    0.077837] ipi_send_cpu: cpu=0 callsite=irq_work_queue+0x22/0x4c callback=srcu_irq_work+0x0/0x58

Is this the last line you can see from your (serial?) console, or we are
missing a few more lines? Maybe it'll should the same hung task
messages?

The ipi_send_cpu message means RCU was able to send an IPI, now if there
is no ipi_handler between the send and the hung task, that would mean
the IPI was missed.

Regards,
Boqun

> Let me know if there is something I'm missing or something else I can do to help
> find the bug.
> 
> /Emil