linux-kernel - Re: [PATCH 26/41] kernel/fork: assert no VMA readers during its destruction

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Message-ID: <Y8bB3TFLQV2HJZQ+@dhcp22.suse.cz>
Date:   Tue, 17 Jan 2023 16:42:21 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [PATCH 26/41] kernel/fork: assert no VMA readers during its
 destruction

On Mon 09-01-23 12:53:21, Suren Baghdasaryan wrote:
> Assert there are no holders of VMA lock for reading when it is about to be
> destroyed.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  include/linux/mm.h | 8 ++++++++
>  kernel/fork.c      | 2 ++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 594e835bad9c..c464fc8a514c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -680,6 +680,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>  	VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
>  }
>  
> +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> +{
> +	VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> +		      vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> +		      vma);

Do we really need to check for vm_lock_seq? rwsem_is_locked should tell
us something is wrong on its own, no? This could be somebody racing with
the vma destruction and using the write lock. Unlikely but I do not see
why to narrow debugging scope.
-- 
Michal Hocko
SUSE Labs