linux-kernel - Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP

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Message-ID: <5a687e9c-f375-d9cc-f781-37cd5bedacb9@oracle.com>
Date:   Mon, 6 Dec 2021 08:39:53 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, linux-kernel@...r.kernel.org,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be
 brought up again.


On 12/6/21 6:25 AM, Sebastian Andrzej Siewior wrote:
> On 2021-11-24 21:17:34 [-0500], Boris Ostrovsky wrote:
>> On 11/24/21 5:54 PM, Thomas Gleixner wrote:
>>> Any comment from XEN folks?
>>
>> If memory allocation in cpu_initialize_context() fails we will not be
>> able to bring up the VCPU because xen_cpu_initialized_map bit at the
>> top of that routine will already have been set. We will BUG in
>> xen_pv_cpu_up() on second (presumably successful) attempt because
>> nothing for that VCPU would be initialized. This can in principle be
>> fixed by moving allocation to the top of the routine and freeing
>> context if the bit in the bitmap is already set.
>>
>>
>> Having said that, allocation really should not fail: for PV guests we
>> first bring max number of VCPUs up and then offline them down to
>> however many need to run. I think if we fail allocation during boot we
>> are going to have a really bad day anyway.
>>
> So can we keep the patch as-is or are some changes needed?


I think for the sake of completeness we could add


diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 6a8f3b53ab83..86368fcef466 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -277,8 +277,11 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
                 return 0;

         ctxt = kzalloc(sizeof(*ctxt), GFP_KERNEL);
-       if (ctxt == NULL)
+       if (ctxt == NULL) {
+               cpumask_clear_cpu(cpu, xen_cpu_initialized_map);
+               cpumask_clear_cpu(cpu, cpu_callout_mask);
                 return -ENOMEM;
+       }

         gdt = get_cpu_gdt_rw(cpu);


-boris