linux-kernel - Re: [PATCH 3.16 084/294] KVM: async_pf: make rcu irq exit if not triggered from idle task

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Date:   Tue, 7 Nov 2017 11:37:33 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     akpm@...ux-foundation.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 3.16 084/294] KVM: async_pf: make rcu irq exit if not
 triggered from idle task

On 07/11/2017 00:03, Ben Hutchings wrote:
> 3.16.50-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> commit 337c017ccdf2653d0040099433fc1a2b1beb5926 upstream.
> 
>  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.  However,
> we need to call the pair even if the function calls schedule(), as seen
> from the above backtrace.
> 
> This patch fixes it by informing the RCU subsystem exit/enter the irq
> towards/away from idle for both n.halted and !n.halted.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> ---
>  arch/x86/kernel/kvm.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -150,6 +150,8 @@ void kvm_async_pf_task_wait(u32 token)
>  		if (hlist_unhashed(&n.link))
>  			break;
>  
> +		rcu_irq_exit();
> +
>  		if (!n.halted) {
>  			local_irq_enable();
>  			schedule();
> @@ -158,11 +160,11 @@ void kvm_async_pf_task_wait(u32 token)
>  			/*
>  			 * We cannot reschedule. So halt.
>  			 */
> -			rcu_irq_exit();
>  			native_safe_halt();
>  			local_irq_disable();
> -			rcu_irq_enter();
>  		}
> +
> +		rcu_irq_enter();
>  	}
>  	if (!n.halted)
>  		finish_wait(&n.wq, &wait);
> 

Looks good, please backport

b862789aa5186d5ea3a024b7cfe0f80c3a38b980 and
a2b7861bb33b2538420bb5d8554153484d3f961f

as well.

Thanks,

Paolo