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Goncalves" , Andrew Morton , Mel Gorman , Michal Hocko , Thomas Gleixner Subject: Re: [PATCH] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save(). Message-ID: <20230623093525.ajhfET8r@linutronix.de> References: <20230621104034.HT6QnNkQ@linutronix.de> <0e9fc992-8e05-2e63-b3b1-d8d3ce89fc16@I-love.SAKURA.ne.jp> <20230621130641.-5iueY1I@linutronix.de> <20230621143421.BgHjJklo@linutronix.de> <01031ffe-c81f-9cec-76fb-e70d548429cf@I-love.SAKURA.ne.jp> <8b6d3f39-c573-ca2b-957b-8c48c2fa68ad@I-love.SAKURA.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: X-Rspamd-Queue-Id: 69B3BA001F X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: nqmnaj61meaqxs9ygq8r8en5693sp477 X-HE-Tag: 1687512928-559426 X-HE-Meta: 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 zlVveQgG 3YB8UBQK4QX9Yfe9lFNLsD6vui6Q7uy8i8KwuTTWFS7N8vI8IswoArVzplU2gASj7aAzwTKRt2U2eTZbaMwfqJX3eibjanilkSWlyd5ZdBx6sa9P1lqxn9l6A0kW91fEzBBUmBeQ+qGNhyz8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2023-06-22 23:28:25 [+0900], Tetsuo Handa wrote: > On 2023/06/22 23:11, Petr Mladek wrote: > >> Question to Peter: > >> Even if local_irq_save(flags) disables IRQ, NMI context can enqueue me= ssage via printk(). > >> When does the message enqueued from NMI context gets printed? > >=20 > > They are flushed to the console either by irq_work or by another > > printk(). The irq_work could not be proceed when IRQs are disabled. >=20 > Is that rule same for both COMFIG_PREEMPT_RT=3Dy and COMFIG_PREEMPT_RT=3D= n ? >=20 > If yes, when Sebastian's patch is applied, IRQs will not be disabled for > COMFIG_PREEMPT_RT=3Dy kernels because write_seqlock_irqsave() is equivale= nt > with write_seqlock() because spin_lock_irqsave(lock, flags) is equivalent > with spin_lock(lock), and So why do you accept that spin_lock_irqsave() does not disable interrupts on PREEMPT_RT but not that printk() is different and has dedicated printing threads? Regarding printk: - dedicated printking thread is used. - in emergcy cases a different driver is used which does not rely on lock held by tty. > > But another non-deferred printk() would try to flush them immediately. > >=20 > >> If there is a possibility > >> that the message enqueued from NMI context gets printed between > >> "write_seqlock_irqsave(&zonelist_update_seq, flags) and printk_deferre= d_enter()" or > >> "printk_deferred_exit() and write_sequnlock_irqrestore(&zonelist_updat= e_seq, flags)" ? > >> If yes, we can't increment zonelist_update_seq.seqcount before printk_= deferred_enter()... > >=20 > > It might happen when a printk() is called in these holes. >=20 > printk() can happen between these holes for COMFIG_PREEMPT_RT=3Dy kernels. > We will need to call printk_deferred_enter() before incrementing > zonelist_update_seq.seqcount in order to close these holes. The only hole is the possible lockdep splat which I'm going to change=E2=80= =A6 Sebastian