linux-kernel - Re: [PATCH v3 06/10] mmap locking API: convert nested write lock sites

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Date:   Wed, 1 Apr 2020 15:42:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michel Lespinasse <walken@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Liam Howlett <Liam.Howlett@...cle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Markus Elfring <Markus.Elfring@....de>
Subject: Re: [PATCH v3 06/10] mmap locking API: convert nested write lock
 sites

On Fri, Mar 27, 2020 at 03:50:58PM -0700, Michel Lespinasse wrote:

> @@ -26,6 +31,12 @@ static inline void mmap_write_unlock(struct mm_struct *mm)
>  	up_write(&mm->mmap_sem);
>  }
>  
> +/* Pairs with mmap_write_lock_nested() */
> +static inline void mmap_write_unlock_nested(struct mm_struct *mm)
> +{
> +	up_write(&mm->mmap_sem);
> +}
> +
>  static inline void mmap_downgrade_write_lock(struct mm_struct *mm)
>  {
>  	downgrade_write(&mm->mmap_sem);

Why does unlock_nested() make sense ?