linux-kernel - Re: [PATCH] mempool: clarify behavior of mempool_alloc

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Message-ID: <20251015192717.HvwzCChE@linutronix.de>
Date: Wed, 15 Oct 2025 21:27:17 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...two.org>,
	David Rientjes <rientjes@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Harry Yoo <harry.yoo@...cle.com>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] mempool: clarify behavior of mempool_alloc_preallocated()

On 2025-10-15 11:52:24 [-0700], Vishal Moola (Oracle) wrote:
> > --- a/mm/mempool.c
> > +++ b/mm/mempool.c
> > @@ -461,8 +461,8 @@ EXPORT_SYMBOL(mempool_alloc_noprof);
> >   *             mempool_create().
> >   *
> >   * This function is similar to mempool_alloc, but it only attempts allocating
> > - * an element from the preallocated elements. It does not sleep and immediately
> > - * returns if no preallocated elements are available.
> > + * an element from the preallocated elements. It only takes a single spinlock_t
> 
> Might it make more sense to say "It may sleep" instead of "takes a
> single spinlock_t"?

May sleep usually refers to something that can not be used in an
interrupt handler.

> I feel like the fact that we take a spinlock isn't the important part
> here (especially because we always drop it before returning).
It actually is. A spinlock_t can not be acquired in hardirq context or
when interrupts are explicitly disabled via local_irq_disable().
Therefore you should use the function in a local_irq_disable() section.

Sebastian