linux-kernel - Re: Do we need to disable preemption in flush_tlb

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Date:   Thu, 15 Mar 2018 10:32:34 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "peterz@...radead.org" <peterz@...radead.org>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: Do we need to disable preemption in flush_tlb_range()?

+CC some more folks for intc/irq insights - please see question at the bottom !

On 03/15/2018 02:39 AM, Alexey Brodkin wrote:
> Hi Peter,
> 
> On Thu, 2018-03-15 at 09:27 +0100, Peter Zijlstra wrote:
>> On Wed, Mar 14, 2018 at 01:19:01PM -0700, Vineet Gupta wrote:
>>> +CC Peter since we have his attention ;-)
>>
>> Yeah, timezone collision there, I typically sleep at 1am ;-)
>>
>>> On 03/01/2018 07:13 AM, Alexey Brodkin wrote:
>>>> Hi Vineet,
>>>>
>>>> Just noticed that in comments for smp_call_function_many() it is said that
>>>> preemption must be disabled during its execution. And that function gets executed
>>>> among other ways like that:
>>>> -------------------------->8-----------------------
>>>>     flush_tlb_range()
>>>>       -> on_each_cpu_mask()
>>>>            -> smp_call_function_many()
>>>> -------------------------->8-----------------------
>>>
>>> In general I prefer not to - Peter what say you ?
>>
>> The comment with smp_call_function_many() is correct, it relies on
>> preemption being disabled in a number of ways. I would expect
>> this_cpu_ptr() for example to complain when used with preemption
>> enabled (CONFIG_DEBUG_PREEMPT).
> 
> I just tried CONFIG_DEBUG_PREEMPT and the only thing I got was that:
> -------------------------->8-----------------------
> ARC perf        : 8 counters (32 bits), 32 conditions, [overflow IRQ support]
> BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
> caller is arc_pmu_device_probe+0x24e/0x29c
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.14.14+ #67
> 
> Stack Trace:
>    arc_unwind_core.constprop.1+0xd0/0xf4
>    dump_stack+0x64/0x7c
>    debug_smp_processor_id+0xb8/0xbc
>    arc_pmu_device_probe+0x24e/0x29c
>    platform_drv_probe+0x26/0x5c
>    really_probe+0x288/0x338
>    __driver_attach+0xc4/0xc8
>    bus_for_each_dev+0x38/0x70
>    bus_add_driver+0x12a/0x18c
>    driver_register+0x50/0xec
>    do_one_initcall+0x32/0x108
>    kernel_init_freeable+0xfe/0x188
> -------------------------->8-----------------------
> 
> That happens because in PMU probe routine we want to
> configure IRQ handlers on all other cores:
> -------------------------->8-----------------------
>    arc_pmu_device_probe() ->
>      on_each_cpu(arc_cpu_pmu_irq_init, &irq, 1): preempt_disable() ->
>        enable_percpu_irq(irq, IRQ_TYPE_NONE) ->
>          smp_processor_id() with disabled preemption.
> -------------------------->8-----------------------
> 
> Which poses another preemption related question - how do IRQ setup on
> all cores properly? :)
> 
> -Alexey
>