linux-kernel - Re: [EXTERNAL] [PATCH 2/2] KVM: x86: disable interrupts while pvclock_gtod_sync

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Date:   Thu, 1 Apr 2021 18:36:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     mtosatti@...hat.com, vkuznets@...hat.com,
        syzbot+b282b65c2c68492df769@...kaller.appspotmail.com
Subject: Re: [EXTERNAL] [PATCH 2/2] KVM: x86: disable interrupts while
 pvclock_gtod_sync_lock is taken

On 01/04/21 17:27, David Woodhouse wrote:
>> -       spin_lock(&ka->pvclock_gtod_sync_lock);
>> +       spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
>>          use_master_clock = ka->use_master_clock;
>>          if (use_master_clock) {
>>                  host_tsc = ka->master_cycle_now;
>>                  kernel_ns = ka->master_kernel_ns;
>>          }
>> -       spin_unlock(&ka->pvclock_gtod_sync_lock);
>> +       spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
>>
>>          /* Keep irq disabled to prevent changes to the clock */
>>          local_irq_save(flags);
> That seems a little gratuitous at the end; restoring the flags as part
> of the spin_unlock_irqrestore() and then immediately calling
> local_irq_save().
> 
> Is something going to complain if we just use spin_unlock() there and
> then later restore the flags with the existing local_irq_restore()?

Yes, I think it breaks on RT kernels.

> Or should we move the local_irq_save() up above the existing
> spin_lock() and leave the spin lock/unlock as they are?

Nope, also breaks on RT (and this one is explicitly called out by 
Documentation/locking/locktypes.rst).  Since it's necessary to use 
spin_lock_irqsave and spin_unlock_irqrestore, one would need flags and 
flags2 variables which is really horrible.

I thought of a similar one which is to move the critical section within 
the IRQ-disabled region:

         local_irq_save(flags);
	...
         spin_lock(&ka->pvclock_gtod_sync_lock);
         use_master_clock = ka->use_master_clock;
         if (use_master_clock) {
                 host_tsc = ka->master_cycle_now;
                 kernel_ns = ka->master_kernel_ns;
         } else {
                 host_tsc = rdtsc();
                 kernel_ns = get_kvmclock_base_ns();
         }
         spin_unlock(&ka->pvclock_gtod_sync_lock);
	...
	local_irq_restore(flags);

... but didn't do it because of RT again.

Paolo