Subject: Re: [PATCH v2] powerpc/lockdep: fix a false positive warning

From: Ingo Molnar <[email-masked]>
Date: Sat, 7 Sep 2019 09:05:05 +0200

* Qian Cai <[email-masked]> wrote:

> The commit 108c14858b9e ("locking/lockdep: Add support for dynamic
> keys") introduced a boot warning on powerpc below, because since the
> commit 2d4f567103ff ("KVM: PPC: Introduce kvm_tmp framework") adds
> kvm_tmp[] into the .bss section and then free the rest of unused spaces
> back to the page allocator.
> 
> kernel_init
>   kvm_guest_init
>     kvm_free_tmp
>       free_reserved_area
>         free_unref_page
>           free_unref_page_prepare
> 
> Later, alloc_workqueue() happens to allocate some pages from there and
> trigger the warning at,
> 
> if (WARN_ON_ONCE(static_obj(key)))
> 
> Fix it by adding a generic helper arch_is_bss_hole() to skip those areas
> in static_obj(). Since kvm_free_tmp() is only done early during the
> boot, just go lockless to make the implementation simple for now.
> 
> WARNING: CPU: 0 PID: 13 at kernel/locking/lockdep.c:1120
> Workqueue: events work_for_cpu_fn
> Call Trace:
>   lockdep_register_key+0x68/0x200
>   wq_init_lockdep+0x40/0xc0
>   trunc_msg+0x385f9/0x4c30f (unreliable)
>   wq_init_lockdep+0x40/0xc0
>   alloc_workqueue+0x1e0/0x620
>   scsi_host_alloc+0x3d8/0x490
>   ata_scsi_add_hosts+0xd0/0x220 [libata]
>   ata_host_register+0x178/0x400 [libata]
>   ata_host_activate+0x17c/0x210 [libata]
>   ahci_host_activate+0x84/0x250 [libahci]
>   ahci_init_one+0xc74/0xdc0 [ahci]
>   local_pci_probe+0x78/0x100
>   work_for_cpu_fn+0x40/0x70
>   process_one_work+0x388/0x750
>   process_scheduled_works+0x50/0x90
>   worker_thread+0x3d0/0x570
>   kthread+0x1b8/0x1e0
>   ret_from_kernel_thread+0x5c/0x7c
> 
> Fixes: 108c14858b9e ("locking/lockdep: Add support for dynamic keys")
> Signed-off-by: Qian Cai <[email-masked]>
> ---
> 
> v2: No need to actually define arch_is_bss_hole() powerpc64 only.
> 
>  arch/powerpc/include/asm/sections.h | 11 +++++++++++
>  arch/powerpc/kernel/kvm.c           |  5 +++++
>  include/asm-generic/sections.h      |  7 +++++++
>  kernel/locking/lockdep.c            |  3 +++
>  4 files changed, 26 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
> index 4a1664a8658d..4f5d69c42017 100644
> --- a/arch/powerpc/include/asm/sections.h
> +++ b/arch/powerpc/include/asm/sections.h
> @@ -5,8 +5,19 @@
>  
>  #include <linux/elf.h>
>  #include <linux/uaccess.h>
> +
> +#define arch_is_bss_hole arch_is_bss_hole
> +
>  #include <asm-generic/sections.h>
>  
> +extern void *bss_hole_start, *bss_hole_end;
> +
> +static inline int arch_is_bss_hole(unsigned long addr)
> +{
> +	return addr >= (unsigned long)bss_hole_start &&
> +	       addr < (unsigned long)bss_hole_end;
> +}
> +
>  extern char __head_end[];
>  
>  #ifdef __powerpc64__
> diff --git a/arch/powerpc/kernel/kvm.c b/arch/powerpc/kernel/kvm.c
> index b7b3a5e4e224..89e0e522e125 100644
> --- a/arch/powerpc/kernel/kvm.c
> +++ b/arch/powerpc/kernel/kvm.c
> @@ -66,6 +66,7 @@
>  static bool kvm_patching_worked = true;
>  char kvm_tmp[1024 * 1024];
>  static int kvm_tmp_index;
> +void *bss_hole_start, *bss_hole_end;
>  
>  static inline void kvm_patch_ins(u32 *inst, u32 new_inst)
>  {
> @@ -707,6 +708,10 @@ static __init void kvm_free_tmp(void)
>  	 */
>  	kmemleak_free_part(&kvm_tmp[kvm_tmp_index],
>  			   ARRAY_SIZE(kvm_tmp) - kvm_tmp_index);
> +
> +	bss_hole_start = &kvm_tmp[kvm_tmp_index];
> +	bss_hole_end = &kvm_tmp[ARRAY_SIZE(kvm_tmp)];
> +
>  	free_reserved_area(&kvm_tmp[kvm_tmp_index],
>  			   &kvm_tmp[ARRAY_SIZE(kvm_tmp)], -1, NULL);
>  }
> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index d1779d442aa5..4d8b1f2c5fd9 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -91,6 +91,13 @@ static inline int arch_is_kernel_initmem_freed(unsigned long addr)
>  }
>  #endif
>  
> +#ifndef arch_is_bss_hole
> +static inline int arch_is_bss_hole(unsigned long addr)
> +{
> +	return 0;
> +}
> +#endif
> +
>  /**
>   * memory_contains - checks if an object is contained within a memory region
>   * @begin: virtual address of the beginning of the memory region
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 4861cf8e274b..cd75b51f15ce 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -675,6 +675,9 @@ static int static_obj(const void *obj)
>  	if (arch_is_kernel_initmem_freed(addr))
>  		return 0;
>  
> +	if (arch_is_bss_hole(addr))
> +		return 0;

arch_is_bss_hole() should use a 'bool' - but other than that, this 
looks good to me, if the PowerPC maintainers agree too.

Thanks,

	Ingo