Re: [PATCH] mm: pgtable: Ensure pml spinlock gets unlock
From: I Hsin Cheng
Date: Mon Feb 10 2025 - 03:20:49 EST
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@xxxxxxxxx>
> > ---
> > 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?
Best regards,
I Hsin Cheng