Re: [uml-devel] [PATCH] Fix for "occasional userspace process in D/Z state" bug

[Richard Weinberger <richard@nod.at>]

Am 26.09.2014 13:49, schrieb anton.ivanov@kot-begemot.co.uk:
> From: Anton Ivanov <antivano@cisco.com>
> 
> This is a fix for a very old UML bug which can be triggered with stock 
> UML. It takes a lot of effort to trigger it there because the 
> lseek()/read() | write() mechanics of the UBD driver implicitly sync the 
> memory all the time by hitting the appropriate barrier implementation in 
> the host kernel. 
> 
> By improving the disk susbsystem we make this bug raise its ugly head
> with a vengeance - you can get a process in D (with an occasional child
> in Z state) simply by running an apt-get on 30-40 large packages. 
> 
> Is this correct place to have the sync - no idea. It may need to move
> to somewhere inside tlb.c. With the fence in exec.c it works (TM).
> 
> If I understand this correctly, this also needs to be an instruction 
> appropriate for the underlying host so just a barrier() will not cut 
> it. You have to fence. En-guarde... Touche... :)
> 
> Signed-off-by: Anton Ivanov <antivano@cisco.com>
> ---
>  arch/um/kernel/exec.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/um/kernel/exec.c b/arch/um/kernel/exec.c
> index 0d7103c..7cb6805 100644
> --- a/arch/um/kernel/exec.c
> +++ b/arch/um/kernel/exec.c
> @@ -27,6 +27,11 @@ void flush_thread(void)
>  	ret = unmap(&current->mm->context.id, 0, STUB_START, 0, &data);
>  	ret = ret || unmap(&current->mm->context.id, STUB_END,
>  			   host_task_size - STUB_END, 1, &data);
> +#ifdef CONFIG_X86_32
> +	alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2);
> +#else
> +	asm volatile("mfence":::"memory");
> +#endif

Why not mb()?
I'm not sure whether this fix is correct.

Thanks,
//richard

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