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Date: Wed, 17 Jan 2018 17:33:10 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] lockdep: Allow checking a read-only lock
Message-ID: <20180117163310.GB2249@hirez.programming.kicks-ass.net>
References: <20180117151414.23686-1-willy@infradead.org>
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On Wed, Jan 17, 2018 at 07:14:11AM -0800, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> I am not for one moment suggesting that the concept of a read-only lock
> makes sense.  You can't sensibly put one in ROM or in read-only mappings.
> What does make sense is some APIs want to specify a const pointer to
> indicate that they do not modify the object being pointed to.  One example
> we have of this today is in the networking stack; tcp_md5_do_lookup takes
> a const struct sock * argument and wants to ensure that the caller either
> took the socket lock or the rcu lock.
> 
> At the moment, tcp_md5_do_lookup() is actually lying to its callers;
> lockdep_sock_is_held() casts away the constness of the pointer because
> lockdep actually does modify the lock when checking whether it's held
> (under rare and unnecessary conditions).
> 
> Fix this situation by (patch 1) only assigning a lock key on registration,
> not on check, (patch 2) marking the pointers in the lockdep check path
> as const and (patch 3) converting a few of the callers to themselves
> be const, removing the nasty hack in lockdep_sock_is_held().
> 

Seems OK.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Ingo can you make that happen?