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Date:   Mon, 12 Nov 2018 16:40:23 -0500
From:   Sasha Levin <sashal@kernel.org>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        dipankar <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        kernel-team <kernel-team@android.com>
Subject: Re: [PATCH tip/core/rcu 0/41] More RCU flavor consolidation cleanup
 for v4.21/v5.0
Message-ID: <20181112214023.GA64029@sasha-vm>
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 <20181112160137.GI4170@linux.ibm.com>
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On Mon, Nov 12, 2018 at 08:01:37AM -0800, Paul E. McKenney wrote:
>On Mon, Nov 12, 2018 at 09:07:50AM -0500, Mathieu Desnoyers wrote:
>> ----- On Nov 11, 2018, at 2:41 PM, paulmck paulmck@linux.ibm.com wrote:
>>
>> > Hello!
>> >
>> > This series does additional cleanup for the RCU flavor consolidation,
>> > focusing primarily on uses of old API members, for example, so that
>> > call_rcu_bh() becomes call_rcu().  There are also a few straggling
>> > internal-to-RCU cleanups.
>> >
>> > 1.	Remove unused rcu_state externs, courtesy of Joel Fernandes.
>> >
>> > 2.	Fix rcu_{node,data} comments about gp_seq_needed, courtesy of
>> >	Joel Fernandes.
>> >
>> > 3.	Eliminate synchronize_rcu_mult() and its sole caller.
>> >
>> > 4.	Consolidate the RCU update functions invoked by sync.c.
>> >
>> > 5-41.	Replace old flavorful RCU API calls with the corresponding
>> >	vanilla calls.
>>
>> Hi Paul,
>>
>> Just a heads up: we might want to spell out warnings in very big letters
>> for anyone trying to backport code using RCU from post-4.21 kernels
>> back to older kernels. I fear that newer code will build just fine
>> on older kernels, but will spectacularly fail in hard-to-debug ways at
>> runtime.
>>
>> Renaming synchronize_rcu() and call_rcu() to something that did not
>> exist in prior kernels would prevent that. It may not be as pretty
>> though.
>
>From v4.20 rather than v4.21, but yes.  Would it make sense to have Sasha
>automatically flag -stable candidates going back past that boundary that
>contain call_rcu(), synchronize_rcu(), etc.?  Adding Sasha on CC, and
>I might be able to touch base with him this week.

We had a similar issue recently with a vfs change
(https://patchwork.kernel.org/patch/10604339/) leading to potentially
the same results as described above, we took it as is to avoid these
issues in the future, though this is a much smaller change than what's
proposed here.

We can look into an good way to solve this. While I can alert on
post-4.20 stable tagged patches that touch rcu, do you really want to be
dealing with this for the next 10+ years? It'll also means each of those
patches will need a manual backport.

Let's talk at Plumbers :)

--
Thanks,
Sasha