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Date:   Thu, 9 Sep 2021 14:33:15 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Yonghong Song <yhs@fb.com>
Cc:     bpf@vger.kernel.org, linux-mm@kvack.org,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, kernel-team@fb.com,
        Luigi Rizzo <lrizzo@google.com>, Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [PATCH bpf-next v4] bpf: fix lockdep warning triggered by
 stack_map_get_build_id_offset()
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On Wed, Sep 08, 2021 at 11:02:45PM -0700, Yonghong Song wrote:
> @@ -204,9 +204,10 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>  	}
>  
>  	if (!work) {
> -		mmap_read_unlock_non_owner(current->mm);
> +		mmap_read_unlock(current->mm);
>  	} else {
>  		work->mm = current->mm;
> +		rwsem_release(&current->mm->mmap_lock.dep_map, _RET_IP_);
>  		irq_work_queue(&work->irq_work);

This needs a comment before the rwsem_release().  Something like:

		/*
		 * The lock will be released once we're out of interrupt
		 * context.  Tell lockdep that we've released it now so
		 * it doesn't complain that we forgot to release it.
		 */