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From: Peter Zijlstra <>
To: Matthew Wilcox <>
Cc: Ingo Molnar <>,
	"David S. Miller" <>,
	Matthew Wilcox <>,
	Thomas Gleixner <>,
Subject: Re: [PATCH 0/3] lockdep: Allow checking a read-only lock
Date: Wed, 17 Jan 2018 17:33:10 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Jan 17, 2018 at 07:14:11AM -0800, Matthew Wilcox wrote:
> From: Matthew Wilcox <>
> 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) <>

Ingo can you make that happen?

      parent reply	other threads:[~2018-01-17 16:33 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-17 15:14 Matthew Wilcox
2018-01-17 15:14 ` [PATCH 1/3] lockdep: Assign lock keys on registration Matthew Wilcox
2018-01-18 11:00   ` [tip:locking/core] " tip-bot for Matthew Wilcox
2018-01-17 15:14 ` [PATCH 2/3] lockdep: Make lockdep checking constant Matthew Wilcox
2018-01-18 11:01   ` [tip:locking/core] " tip-bot for Matthew Wilcox
2018-01-17 15:14 ` [PATCH 3/3] lockdep: Convert some users to const Matthew Wilcox
2018-01-18 11:01   ` [tip:locking/core] " tip-bot for Matthew Wilcox
2018-01-17 16:33 ` Peter Zijlstra [this message]

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