linux-kernel - Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard policy

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Date:	Fri, 19 Feb 2016 16:04:35 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Yuki Shibuya <shibuya.yk@...s.nec.co.jp>
Subject: Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard
 policy

2016-02-18 19:08+0100, Paolo Bonzini:
> On 17/02/2016 20:14, Radim Krčmář wrote:
>> +	/* pit->pit_state.lock was overloaded to prevent userspace from getting
>> +	 * an inconsistent state after running multiple KVM_REINJECT_CONTROL
>> +	 * ioctls in parallel.  Use a separate lock if that ioctl isn't rare.
>> +	 */
>> +	mutex_lock(&pit->pit_state.lock);
>> +	kvm_pit_set_reinject(pit, control->pit_reinject);
>> +	mutex_unlock(&pit->pit_state.lock);
> 
> ... so in patch 7 concurrent _writes_ of reinject are protected by the
> lock, but reads are done outside it (in pit_timer_fn).  WDYT about
> making reinject an atomic_t?

There was/is no harm in having reinject non-atomic.  This patch added
notifiers, which is the reason for re-introducing a mutex.

Userspace can (and SHOULDN'T) call this function multiple times,
concurrently, so the mutex prevents a situations where, e.g. only one
notifier is registered in the end.

I thought about really stupid stuff when doing this series ...