linux-kernel - Re: [PATCH] mm: pgtable: Ensure pml spinlock gets unlock

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Message-ID: <e5d5bafc-7809-42c2-ba8d-554b9667d00c@bytedance.com>
Date: Mon, 10 Feb 2025 16:31:59 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: I Hsin Cheng <richard120310@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: pgtable: Ensure pml spinlock gets unlock



On 2025/2/10 16:20, I Hsin Cheng wrote:
> On Mon, Feb 10, 2025 at 12:05:05PM +0800, Qi Zheng wrote:
>>
>>
>> On 2025/2/9 02:49, I Hsin Cheng wrote:
>>> When !start_pte is true, the "pml" spinlock is still being holded and
>>> the branch "out_pte" is taken. If "ptl" is equal to "pml", the lock
>>> "pml" will still be locked when the function returns.
>>
>> No. When start_pte is NULL, the ptl must also be NULL, so the ptl and
>> pml will not be equal.
>>
>>>
>>> It'll be better to set a new branch "out_pte" and jump to it when
>>> !start_pte is true at the first place, therefore no additional check for
>>> "start_pte" or "ptl != pml" is needed, simply unlock "pml" and return.
>>>
>>> Signed-off-by: I Hsin Cheng <richard120310@...il.com>
>>> ---
>>>    mm/pt_reclaim.c | 6 +++++-
>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
>>> index 7e9455a18aae..163e38f1728d 100644
>>> --- a/mm/pt_reclaim.c
>>> +++ b/mm/pt_reclaim.c
>>> @@ -43,7 +43,7 @@ void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
>>>    	pml = pmd_lock(mm, pmd);
>>>    	start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pmdval, &ptl);
>>>    	if (!start_pte)
>>> -		goto out_ptl;
>>> +		goto out_pte;
>>>    	if (ptl != pml)
>>>    		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>>> @@ -68,4 +68,8 @@ void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
>>>    		pte_unmap_unlock(start_pte, ptl);
>>>    	if (ptl != pml)
>>>    		spin_unlock(pml);
>>> +	return;
>>> +
>>> +out_pte:
>>> +	spin_unlock(pml);
>>>    }
> 
> Hi Qi,
> 
> Thanks for your kindly review!
> 
>> No. When start_pte is NULL, the ptl must also be NULL, so the ptl and
>> pml will not be equal.
> 
> Since this is the case, we don't have to do any addtional check for
> "start_pte" and "ptl != pml", we can be sure that only the second check
> will be true so we can unlock "pml" without the redundant check, that's
> my understanding, what do you think?
Adding a label

vs

Redundant check in rare cases

Not sure if this is worth it. ;)

> 
> Best regards,
> I Hsin Cheng
> 
>