linux-kernel - Re: [PATCH v2 2/6] futex: Use RCU-based per-CPU reference counting instead of rcuref

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Message-ID: <188efc4a8e37bc5dc64dd18989f9a254ef218449.camel@linaro.org>
Date: Fri, 01 Aug 2025 15:59:06 +0100
From: André Draszik <andre.draszik@...aro.org>
To: Thomas Gleixner <tglx@...utronix.de>, Sebastian Andrzej Siewior
	 <bigeasy@...utronix.de>, linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>, Darren Hart	
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar	
 <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra	
 <peterz@...radead.org>, Valentin Schneider <vschneid@...hat.com>, Waiman
 Long	 <longman@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Lorenzo Stoakes	 <lorenzo.stoakes@...cle.com>,
 Michal Hocko <mhocko@...e.com>, Mike Rapoport	 <rppt@...nel.org>, Suren
 Baghdasaryan <surenb@...gle.com>, Vlastimil Babka	 <vbabka@...e.cz>,
 linux-mm@...ck.org
Subject: Re: [PATCH v2 2/6] futex: Use RCU-based per-CPU reference counting
 instead of rcuref_t

On Wed, 2025-07-30 at 21:44 +0200, Thomas Gleixner wrote:
> On Wed, Jul 30 2025 at 13:20, André Draszik wrote:
> > kmemleak complains about a new memleak with this commit:
> > 
> > [  680.179004][  T101] kmemleak: 1 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
> > 
> > $ cat /sys/kernel/debug/kmemleak
> > unreferenced object (percpu) 0xc22ec0eface8 (size 4):
> >   comm "swapper/0", pid 1, jiffies 4294893115
> >   hex dump (first 4 bytes on cpu 7):
> >     01 00 00 00                                      ....
> >   backtrace (crc b8bc6765):
> >     kmemleak_alloc_percpu+0x48/0xb8
> >     pcpu_alloc_noprof+0x6ac/0xb68
> >     futex_mm_init+0x60/0xe0
> >     mm_init+0x1e8/0x3c0
> >     mm_alloc+0x5c/0x78
> >     init_args+0x74/0x4b0
> >     debug_vm_pgtable+0x60/0x2d8
> > 
> > Reverting this commit (and patches 3 and 4 in this series due to context),
> > makes kmemleak happy again.
> 
> Unsurprisingly ...
> 
> debug_vm_pgtable() allocates it via mm_alloc() -> mm->init() and then
> after the selftest it invokes mmdrop(), which does not free it, as it is
> only freed in __mmput().
> 
> The patch below should fix it.

It does. Thanks Thomas!

A.