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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Qian Cai <cai@lca.pw>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        lenb@kernel.org, Peter Zijlstra <peterz@infradead.org>,
        linux-acpi@vger.kernel.org,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86/acpi: fix a deadlock with cpu hotplug
Date:   Fri, 03 Apr 2020 15:43:50 +0200
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On Friday, April 3, 2020 1:18:07 PM CEST Qian Cai wrote:
> 
> > On Apr 3, 2020, at 5:29 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > 
> > On Sunday, March 29, 2020 4:21:09 PM CEST Qian Cai wrote:
> >> Similar to the commit 0266d81e9bf5 ("acpi/processor: Prevent cpu hotplug
> >> deadlock") except this is for acpi_processor_ffh_cstate_probe():
> >> 
> >> "The problem is that the work is scheduled on the current CPU from the
> >> hotplug thread associated with that CPU.
> >> 
> >> It's not required to invoke these functions via the workqueue because
> >> the hotplug thread runs on the target CPU already.
> >> 
> >> Check whether current is a per cpu thread pinned on the target CPU and
> >> invoke the function directly to avoid the workqueue."
> >> 
> >> Since CONFIG_ACPI_PROCESSOR (for cstate.c) selects
> >> CONFIG_ACPI_CPU_FREQ_PSS (for processor_throttling.c) on x86, just
> >> make call_on_cpu() a static inline function from processor_throttling.c
> >> and use it in cstate.c.
> >> 
> >> WARNING: possible circular locking dependency detected
> >> ------------------------------------------------------
> >> cpuhp/1/15 is trying to acquire lock:
> >> ffffc90003447a28 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: __flush_work+0x4c6/0x630
> >> 
> >> but task is already holding lock:
> >> ffffffffafa1c0e8 (cpuidle_lock){+.+.}-{3:3}, at: cpuidle_pause_and_lock+0x17/0x20
> >> 
> >> which lock already depends on the new lock.
> >> 
> >> the existing dependency chain (in reverse order) is:
> >> 
> >> -> #1 (cpu_hotplug_lock){++++}-{0:0}:
> >> cpus_read_lock+0x3e/0xc0
> >> irq_calc_affinity_vectors+0x5f/0x91
> >> __pci_enable_msix_range+0x10f/0x9a0
> >> pci_alloc_irq_vectors_affinity+0x13e/0x1f0
> >> pci_alloc_irq_vectors_affinity at drivers/pci/msi.c:1208
> >> pqi_ctrl_init+0x72f/0x1618 [smartpqi]
> >> pqi_pci_probe.cold.63+0x882/0x892 [smartpqi]
> >> local_pci_probe+0x7a/0xc0
> >> work_for_cpu_fn+0x2e/0x50
> >> process_one_work+0x57e/0xb90
> >> worker_thread+0x363/0x5b0
> >> kthread+0x1f4/0x220
> >> ret_from_fork+0x27/0x50
> >> 
> >> -> #0 ((work_completion)(&wfc.work)){+.+.}-{0:0}:
> >> __lock_acquire+0x2244/0x32a0
> >> lock_acquire+0x1a2/0x680
> >> __flush_work+0x4e6/0x630
> >> work_on_cpu+0x114/0x160
> >> acpi_processor_ffh_cstate_probe+0x129/0x250
> >> acpi_processor_evaluate_cst+0x4c8/0x580
> >> acpi_processor_get_power_info+0x86/0x740
> >> acpi_processor_hotplug+0xc3/0x140
> >> acpi_soft_cpu_online+0x102/0x1d0
> >> cpuhp_invoke_callback+0x197/0x1120
> >> cpuhp_thread_fun+0x252/0x2f0
> >> smpboot_thread_fn+0x255/0x440
> >> kthread+0x1f4/0x220
> >> ret_from_fork+0x27/0x50
> >> 
> >> other info that might help us debug this:
> >> 
> >> Chain exists of:
> >> (work_completion)(&wfc.work) --> cpuhp_state-up --> cpuidle_lock
> >> 
> >> Possible unsafe locking scenario:
> >> 
> >> CPU0                    CPU1
> >> ----                    ----
> >> lock(cpuidle_lock);
> >>                         lock(cpuhp_state-up);
> >>                         lock(cpuidle_lock);
> >> lock((work_completion)(&wfc.work));
> >> 
> >> *** DEADLOCK ***
> >> 
> >> 3 locks held by cpuhp/1/15:
> >> #0: ffffffffaf51ab10 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x69/0x2f0
> >> #1: ffffffffaf51ad40 (cpuhp_state-up){+.+.}-{0:0}, at: cpuhp_thread_fun+0x69/0x2f0
> >> #2: ffffffffafa1c0e8 (cpuidle_lock){+.+.}-{3:3}, at: cpuidle_pause_and_lock+0x17/0x20
> >> 
> >> Call Trace:
> >> dump_stack+0xa0/0xea
> >> print_circular_bug.cold.52+0x147/0x14c
> >> check_noncircular+0x295/0x2d0
> >> __lock_acquire+0x2244/0x32a0
> >> lock_acquire+0x1a2/0x680
> >> __flush_work+0x4e6/0x630
> >> work_on_cpu+0x114/0x160
> >> acpi_processor_ffh_cstate_probe+0x129/0x250
> >> acpi_processor_evaluate_cst+0x4c8/0x580
> >> acpi_processor_get_power_info+0x86/0x740
> >> acpi_processor_hotplug+0xc3/0x140
> >> acpi_soft_cpu_online+0x102/0x1d0
> >> cpuhp_invoke_callback+0x197/0x1120
> >> cpuhp_thread_fun+0x252/0x2f0
> >> smpboot_thread_fn+0x255/0x440
> >> kthread+0x1f4/0x220
> >> ret_from_fork+0x27/0x50
> >> 
> >> Signed-off-by: Qian Cai <cai@lca.pw>
> >> ---
> >> 
> >> v2:
> >> Make call_on_cpu() a static inline function to avoid a compilation
> >> error when ACPI_PROCESSOR=m thanks to lkp@intel.com.
> >> 
> >> arch/x86/kernel/acpi/cstate.c       |  3 ++-
> >> drivers/acpi/processor_throttling.c |  7 -------
> >> include/acpi/processor.h            | 10 ++++++++++
> >> 3 files changed, 12 insertions(+), 8 deletions(-)
> >> 
> >> diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
> >> index caf2edccbad2..49ae4e1ac9cd 100644
> >> --- a/arch/x86/kernel/acpi/cstate.c
> >> +++ b/arch/x86/kernel/acpi/cstate.c
> >> @@ -161,7 +161,8 @@ int acpi_processor_ffh_cstate_probe(unsigned int cpu,
> >> 
> >> 	/* Make sure we are running on right CPU */
> >> 
> >> -	retval = work_on_cpu(cpu, acpi_processor_ffh_cstate_probe_cpu, cx);
> >> +	retval = call_on_cpu(cpu, acpi_processor_ffh_cstate_probe_cpu, cx,
> >> +			     false);
> >> 	if (retval == 0) {
> >> 		/* Use the hint in CST */
> >> 		percpu_entry->states[cx->index].eax = cx->address;
> >> diff --git a/drivers/acpi/processor_throttling.c b/drivers/acpi/processor_throttling.c
> >> index 532a1ae3595a..a0bd56ece3ff 100644
> >> --- a/drivers/acpi/processor_throttling.c
> >> +++ b/drivers/acpi/processor_throttling.c
> >> @@ -897,13 +897,6 @@ static long __acpi_processor_get_throttling(void *data)
> >> 	return pr->throttling.acpi_processor_get_throttling(pr);
> >> }
> >> 
> >> -static int call_on_cpu(int cpu, long (*fn)(void *), void *arg, bool direct)
> >> -{
> >> -	if (direct || (is_percpu_thread() && cpu == smp_processor_id()))
> >> -		return fn(arg);
> >> -	return work_on_cpu(cpu, fn, arg);
> >> -}
> >> -
> >> static int acpi_processor_get_throttling(struct acpi_processor *pr)
> >> {
> >> 	if (!pr)
> >> diff --git a/include/acpi/processor.h b/include/acpi/processor.h
> >> index 47805172e73d..770d226b22f2 100644
> >> --- a/include/acpi/processor.h
> >> +++ b/include/acpi/processor.h
> >> @@ -297,6 +297,16 @@ static inline void acpi_processor_ffh_cstate_enter(struct acpi_processor_cx
> >> }
> >> #endif
> >> 
> >> +#ifdef CONFIG_ACPI_CPU_FREQ_PSS
> > 
> > Why does this depend on CONFIG_ACPI_CPU_FREQ_PSS?
> 
> call_on_cpu() was only used in processor_throttling.c which has,
> 
> processor-$(CONFIG_ACPI_CPU_FREQ_PSS)   += processor_throttling.o
> 
> after this patch, it will also be used in cstate.c which has,
> 
> ifneq ($(CONFIG_ACPI_PROCESSOR),)
> obj-y                           += cstate.o
> endif
> 
> i.e.,
> 
> config ACPI_PROCESSOR
>         tristate "Processor"
>         depends on X86 || IA64 || ARM64
>         select ACPI_PROCESSOR_IDLE
>         select ACPI_CPU_FREQ_PSS if X86 || IA64
> 
> Therefore, call_on_cpu() is only used when CONFIG_ACPI_CPU_FREQ_PSS=y.

While technically kind of correct, this is also rather far from straightforward, because
cstate.o and ACPI_CPU_FREQ_PSS are different things logically.

> The #ifdef is rather a safe net that in the future, if we decided to make call_on_cpu()
> a  non-inline function, it will prevent triggering an compilation warning for unused
> function when CONFIG_ACPI_CPU_FREQ_PSS=n.

But as long as it is static inline, the #ifdef isn't necessary, is it?

> It may also serve as a documentation purpose to indicate that function is only used
> with  CONFIG_ACPI_CPU_FREQ_PSS=y.

Which is incidental and therefore misleading.

> > 
> >> +static inline int call_on_cpu(int cpu, long (*fn)(void *), void *arg,
> >> +			      bool direct)
> >> +{
> >> +	if (direct || (is_percpu_thread() && cpu == smp_processor_id()))
> >> +		return fn(arg);
> >> +	return work_on_cpu(cpu, fn, arg);
> >> +}
> >> +#endif
> >> +
> >> /* in processor_perflib.c */
> >> 
> >> #ifdef CONFIG_CPU_FREQ
>