linux-kernel - Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock

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Date:   Thu, 10 Feb 2022 10:33:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        Radoslaw Burny <rburny@...gle.com>, Tejun Heo <tj@...nel.org>,
        rcu <rcu@...r.kernel.org>, cgroups <cgroups@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        paulmck <paulmck@...nel.org>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)

On Wed, Feb 09, 2022 at 03:17:38PM -0500, Waiman Long wrote:
> 
> On 2/9/22 14:45, Namhyung Kim wrote:
> > On Wed, Feb 9, 2022 at 11:28 AM Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.com> wrote:
> > > ----- On Feb 9, 2022, at 2:22 PM, Namhyung Kim namhyung@...nel.org wrote:
> > > > I'm also concerning dynamic allocated locks in a data structure.
> > > > If we keep the info in a hash table, we should delete it when the
> > > > lock is gone.  I'm not sure we have a good place to hook it up all.
> > > I was wondering about this use case as well. Can we make it mandatory to
> > > declare the lock "class" (including the name) statically, even though the
> > > lock per-se is allocated dynamically ? Then the initialization of the lock
> > > embedded within the data structure would simply refer to the lock class
> > > definition.
> > Isn't it still the same if we have static lock classes that the entry needs
> > to be deleted from the hash table when it frees the data structure?
> > I'm more concerned about free than alloc as there seems to be no
> > API to track that in a place.
> 
> We may have to invent some new APIs to do that. For example,
> spin_lock_exit() can be the counterpart of spin_lock_init() and so on. Of
> course, existing kernel code have to be modified to designate the point
> after which a lock is no longer being used or is freed.

The canonical name is _destroy(). We even have mutex_destroy() except
it's usage isn't mandatory.

The easy way out is doing as lockdep does and hook into the memory
allocators and check every free'd hunk of memory for a lock. It does
hoever mean your data structure of choice needs to be able to answer: do
I have an entry in @range. Which mostly disqualifies a hash-table.

Still, I really don't think you need any of this, it's just bloat. A
very limited stack unwind for one of the two tracepoints should allow
you to find the offending lock just fine.