Re: BUG: sleeping function called from invalid context at mm/slub.c:793

[Jan Kiszka <jan.kiszka@siemens.com>]

Am 11.01.2011 11:29, Avi Kivity wrote:
> Please try out the attached patch.
> 
> From f3a6041b5bb3bf7c88f9694a66d7f34be2f78845 Mon Sep 17 00:00:00 2001
> From: Avi Kivity <avi@redhat.com>
> Date: Tue, 11 Jan 2011 12:15:54 +0200
> Subject: [PATCH] KVM: Initialize fpu state in preemptible context
> 
> init_fpu() (which is indirectly called by the fpu switching code) assumes
> it is in process context.  Rather than makeing init_fpu() use an atomic
> allocation, which can cause a task to be killed, make sure the fpu is
> already initialized when we enter the run loop.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kernel/i387.c |    1 +
>  arch/x86/kvm/x86.c     |    3 +++
>  2 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
> index 58bb239..e60c38c 100644
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -169,6 +169,7 @@ int init_fpu(struct task_struct *tsk)
>  	set_stopped_child_used_math(tsk);
>  	return 0;
>  }
> +EXPORT_SYMBOL_GPL(init_fpu);
>  
>  /*
>   * The xstateregs_active() routine is the same as the fpregs_active() routine,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8652643..fd93cda 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5351,6 +5351,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  	int r;
>  	sigset_t sigsaved;
>  
> +	if (!tsk_used_math(current) && init_fpu(current))
> +		return -ENOMEM;
> +

Could become a rainy day for the kvm-kmod maintainer:

For compat support on kernels without init_fpu exported yet, can I
trigger the same result by simply issuing an FPU instruction here so
that do_device_not_available will perform the allocation? Not really
nice, but it doesn't appear to me like there is any code path that would
complain about in-kernel FPU usage (provided we don't need math
emulation - which is quite likely).

Jan

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