linux-kernel - Re: [PATCH] mm: ksm: fix data-race in __ksm_enter / run

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Date:   Thu, 11 Aug 2022 16:00:20 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Abhishek Shah <abhishek.shah@...umbia.edu>,
        Gabriel Ryan <gabe@...columbia.edu>
Subject: Re: [PATCH] mm: ksm: fix data-race in __ksm_enter / run_store

On Tue, 2 Aug 2022 23:15:50 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:

> Abhishek reported a data-race issue,

OK, but it would be better to perform an analysis of the alleged bug,
describe the potential effects if the race is hit, etc.

> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2507,6 +2507,7 @@ int __ksm_enter(struct mm_struct *mm)
>  {
>  	struct mm_slot *mm_slot;
>  	int needs_wakeup;
> +	bool ksm_run_unmerge;
>  
>  	mm_slot = alloc_mm_slot();
>  	if (!mm_slot)
> @@ -2515,6 +2516,10 @@ int __ksm_enter(struct mm_struct *mm)
>  	/* Check ksm_run too?  Would need tighter locking */
>  	needs_wakeup = list_empty(&ksm_mm_head.mm_list);
>  
> +	mutex_lock(&ksm_thread_mutex);
> +	ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
> +	mutex_unlock(&ksm_thread_mutex);
>
>  	spin_lock(&ksm_mmlist_lock);
>  	insert_to_mm_slots_hash(mm, mm_slot);
>  	/*
> @@ -2527,7 +2532,7 @@ int __ksm_enter(struct mm_struct *mm)
>  	 * scanning cursor, otherwise KSM pages in newly forked mms will be
>  	 * missed: then we might as well insert at the end of the list.
>  	 */
> -	if (ksm_run & KSM_RUN_UNMERGE)
> +	if (ksm_run_unmerge)

run_store() can alter ksm_run right here, so __ksm_enter() is still
acting on the old setting?

>  		list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
>  	else
>  		list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);