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Date:   Thu, 8 Aug 2019 11:18:22 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas =?iso-8859-1?Q?Hellstr=F6m?= <thomas@shipmail.org>,
        Jerome Glisse <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Steven Price <steven.price@arm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] pagewalk: use lockdep_assert_held for locking
 validation
Message-ID: <20190808181822.GK5482@bombadil.infradead.org>
References: <20190808154240.9384-1-hch@lst.de>
 <20190808154240.9384-4-hch@lst.de>
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On Thu, Aug 08, 2019 at 06:42:40PM +0300, Christoph Hellwig wrote:
> Use lockdep to check for held locks instead of using home grown
> asserts.
> 
> @@ -319,7 +319,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
>  	if (!walk.mm)
>  		return -EINVAL;
>  
> -	VM_BUG_ON_MM(!rwsem_is_locked(&walk.mm->mmap_sem), walk.mm);
> +	lockdep_assert_held(&walk.mm->mmap_sem);

It occurs to me that this is exactly the pattern that lockdep_pin_lock()
was designed for.  I'm pretty sure things will go badly if any callee
unlocks then relocks the lock.