linux-kernel - Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr

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Date:   Sat, 25 Sep 2021 14:08:42 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org
Cc:     Sohil Mehta <sohil.mehta@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall

On Fri, Sep 24 2021 at 13:04, Thomas Gleixner wrote:
> On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
>> +int uintr_receiver_wait(void)
>> +{
>> +	struct uintr_upid_ctx *upid_ctx;
>> +	unsigned long flags;
>> +
>> +	if (!is_uintr_receiver(current))
>> +		return -EOPNOTSUPP;
>> +
>> +	upid_ctx = current->thread.ui_recv->upid_ctx;
>> +	upid_ctx->upid->nc.nv = UINTR_KERNEL_VECTOR;
>> +	upid_ctx->waiting = true;
>> +	spin_lock_irqsave(&uintr_wait_lock, flags);
>> +	list_add(&upid_ctx->node, &uintr_wait_list);
>> +	spin_unlock_irqrestore(&uintr_wait_lock, flags);
>> +
>> +	set_current_state(TASK_INTERRUPTIBLE);
>
> Because we have not enough properly implemented wait primitives you need
> to open code one which is blantantly wrong vs. a concurrent wake up?
>
>> +	schedule();
>
> How is that correct vs. a spurious wakeup? What takes care that the
> entry is removed from the list?
>
> Again. We have proper wait primitives.

Aisde of that this is completely broken vs. CPU hotplug.

CPUX
  switchto(tsk)
    tsk->upid.ndst = apicid(smp_processor_id();

  ret_to_user()
  ...
  sys_uintr_wait()
    ...
    schedule()

After that CPU X is unplugged which means the task won't be woken up by
an user IPI which is issued after CPU X went down.

Thanks,

        tglx