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

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Message-ID: <aO_taH9CKxmJPnhV@fedora>
Date: Wed, 15 Oct 2025 11:52:24 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: 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>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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 Tue, Oct 14, 2025 at 02:17:23PM +0200, Thomas Weißschuh wrote:
> The documentation of that function promises to never sleep.
> However on PREEMPT_RT a spinlock_t might in fact sleep.
> 
> Reword the documentation so users can predict its behavior better.
> 
> mempool could also replace spinlock_t with raw_spinlock_t which doesn't
> sleep even on PREEMPT_RT but that would take away the improved
> preemptibility of sleeping locks.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> ---
>  mm/mempool.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> index 1c38e873e546fadcc594f041874eb42774e3df16..cceb09b75ebe35f263a5fb95ff6d400221ecbdd5 100644
> --- 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"?

I feel like the fact that we take a spinlock isn't the important part
here (especially because we always drop it before returning).

> + * and immediately returns if no preallocated elements are available.
>   *
>   * Return: pointer to the allocated element or %NULL if no elements are
>   * available.
> 
> ---
> base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> change-id: 20251014-mempool-doc-625dd4996110
> 
> Best regards,
> -- 
> Thomas Weißschuh <thomas.weissschuh@...utronix.de>
>