linux-kernel - Re: [PATCH RFC v2 03/10] locking/local_lock: Introduce localtry_lock

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Message-ID: <CAADnVQL_3dayDBboTkUs6g4PmEOpFhufx736eaBqoqjZcnUwiQ@mail.gmail.com>
Date: Tue, 18 Feb 2025 10:41:08 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Suren Baghdasaryan <surenb@...gle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Christoph Lameter <cl@...ux.com>, 
	David Rientjes <rientjes@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Hyeonggon Yoo <42.hyeyoo@...il.com>, Uladzislau Rezki <urezki@...il.com>, linux-mm <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org, 
	maple-tree@...ts.infradead.org, Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH RFC v2 03/10] locking/local_lock: Introduce localtry_lock_t

On Mon, Feb 17, 2025 at 6:35 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/17/25 15:19, Sebastian Andrzej Siewior wrote:
> > On 2025-02-14 17:27:39 [+0100], Vlastimil Babka wrote:
> >> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> >>
> >> In !PREEMPT_RT local_lock_irqsave() disables interrupts to protect
> >> critical section, but it doesn't prevent NMI, so the fully reentrant
> >> code cannot use local_lock_irqsave() for exclusive access.
> >>
> >> Introduce localtry_lock_t and localtry_lock_irqsave() that
> >> disables interrupts and sets acquired=1, so localtry_lock_irqsave()
> >> from NMI attempting to acquire the same lock will return false.
> >>
> >> In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().
> >> Map localtry_lock_irqsave() to preemptible spin_trylock().
> >> When in hard IRQ or NMI return false right away, since
> >> spin_trylock() is not safe due to PI issues.
> >
> > spin_trylock() is not safe due to explicit locking in the underneath
> > rt_spin_trylock() implementation. Removing this explicit locking and
> > attempting only "trylock" is undesired due to PI implications.

Makes sense.

> Just to be sure, you're suggesting how to reword that sentence in the
> changelog to make it more precise right?
> Alexei will you incorporate that in your version?

Sure. Let's squash patches 3 and 4 and add above
commit log clarification.
Whoever respins first can do it.