linux-kernel - Re: [PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

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Message-ID: <20170801204216.GD316@flask>
Date:   Tue, 1 Aug 2017 22:42:17 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v4] KVM: async_pf: make rcu irq exit if not triggered
 from idle task

2017-08-01 14:22+0200, Paolo Bonzini:
> On 01/08/2017 14:20, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpeng.li@...mail.com>
> > 
> >  WARNING: CPU: 5 PID: 1242 at kernel/rcu/tree_plugin.h:323 rcu_note_context_switch+0x207/0x6b0
> >  CPU: 5 PID: 1242 Comm: unity-settings- Not tainted 4.13.0-rc2+ #1
> >  RIP: 0010:rcu_note_context_switch+0x207/0x6b0
> >  Call Trace:
> >   __schedule+0xda/0xba0
> >   ? kvm_async_pf_task_wait+0x1b2/0x270
> >   schedule+0x40/0x90
> >   kvm_async_pf_task_wait+0x1cc/0x270
> >   ? prepare_to_swait+0x22/0x70
> >   do_async_page_fault+0x77/0xb0
> >   ? do_async_page_fault+0x77/0xb0
> >   async_page_fault+0x28/0x30
> >  RIP: 0010:__d_lookup_rcu+0x90/0x1e0
> > 
> > I encounter this when trying to stress the async page fault in L1 guest w/ 
> > L2 guests running.
> > 
> > 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 not accurate anymore:
> 
> 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.  However,
> we need to call the pair even if the function calls schedule(), as seen
> from the above backtrace.
> 
> Can be fixed when committing.

Done that, thanks.