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

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Message-Id: <20251014-mempool-doc-v1-1-bc9ebf169700@linutronix.de>
Date: Tue, 14 Oct 2025 14:17:23 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: 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>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
 linux-rt-devel@...ts.linux.dev, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH] mempool: clarify behavior of mempool_alloc_preallocated()

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
+ * 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>