Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <3a4ad958-a9a5-c367-a16d-bd89a173a628@I-love.SAKURA.ne.jp>
Date:   Mon, 26 Jun 2023 20:26:02 +0900
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.12.0
Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking
 in do_write_seqcount_begin_nested()
Content-Language: en-US
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        John Ogness <john.ogness@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>, Petr Mladek <pmladek@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
References: <20230623171232.892937-1-bigeasy@linutronix.de>
 <20230623171232.892937-2-bigeasy@linutronix.de>
 <d9b7c170-ed0d-5d37-e099-20d233115943@I-love.SAKURA.ne.jp>
 <20230626081254.XmorFrhs@linutronix.de>
 <0a0c768c-227d-c0cd-1b91-5a884d161c1b@I-love.SAKURA.ne.jp>
 <20230626104831.GT4253@hirez.programming.kicks-ass.net>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
In-Reply-To: <20230626104831.GT4253@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 2023/06/26 19:48, Peter Zijlstra wrote:
> On Mon, Jun 26, 2023 at 06:25:56PM +0900, Tetsuo Handa wrote:
>> On 2023/06/26 17:12, Sebastian Andrzej Siewior wrote:
>>> On 2023-06-24 15:54:12 [+0900], Tetsuo Handa wrote:
>>>> Why not to do the same on the end side?
>>>>
>>>>  static inline void do_write_seqcount_end(seqcount_t *s)
>>>>  {
>>>> - 	seqcount_release(&s->dep_map, _RET_IP_);
>>>>  	do_raw_write_seqcount_end(s);
>>>> +	seqcount_release(&s->dep_map, _RET_IP_);
>>>>  }
>>>
>>> I don't have a compelling argument for doing it. It is probably better
>>> to release the lock from lockdep's point of view and then really release
>>> it (so it can't be acquired before it is released).
>>
>> We must do it because this is a source of possible printk() deadlock.
>> Otherwise, I will nack on PATCH 2/2.
> 
> Don't be like that... just hate on prink like the rest of us. In fact,
> i've been patching out the actual printk code for years because its
> unusable garbage.
> 
> Will this actually still be a problem once all the fancy printk stuff
> lands? That shouldn't do synchronous prints except to 'atomic' consoles
> by default IIRC.

Commit 1007843a9190 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq
seqlock") was applied to 4.14-stable trees, and CONFIG_PREEMPT_RT is available
since 5.3. Thus, we want a fix which can be applied to 5.4-stable and later.
This means that we can't count on all the fancy printk stuff being available.