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

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Message-ID: <20250217141956._01BcsrG@linutronix.de>
Date: Mon, 17 Feb 2025 15:19:56 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: 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@...ck.org,
	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 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.

> Note there is no need to use local_inc for acquired variable,
> since it's a percpu variable with strict nesting scopes.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Other than that, thank you two ;)

Sebastian