linux-kernel - Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle task

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Date:   Tue, 1 Aug 2017 12:37:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] KVM: async_pf: rcu irq exit if not triggered from idle
 task

On 27/07/2017 11:05, Wanpeng Li wrote:
> Commit 9b132fbe5419 (Add rcu user eqs exception hooks for async page fault) 
> adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu idle eqs 
> when needed, to protect the code that needs use rcu. There is no need to call 
> this pairs if async page fault is not triggered from idle task.

This is true, but I think the patch is making things more complex where
it could be simplifying them.  Right now, the "native_safe_halt" branch
is calling rcu_irq_exit/enter but the "schedule" branch is not.  Could
you just pull rcu_irq_exit/enter outside the "if", so that you inform
the RCU subsystem even in the !n.halted case?

Paolo