linux-kernel - locking/local_lock, mm: sparse warnings about shadowed variable

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Message-ID: <CAFGhKbwVyxCwYSNrPaQ-GkuP008+uvDg-wNA5syWLLzODCfpcA@mail.gmail.com>
Date: Wed, 11 Jun 2025 19:37:10 +0200
From: Charlemagne Lasse <charlemagnelasse@...il.com>
To: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Alexei Starovoitov <ast@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, Jakub Kicinski <kuba@...nel.org>, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, 
	LKML <linux-kernel@...r.kernel.org>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-rt-devel@...ts.linux.dev
Subject: locking/local_lock, mm: sparse warnings about shadowed variable

HI,

when I run `make C=2 mm/mlock.o CHECK="sparse -Wshadow"`, I get a lot of

./include/linux/local_lock.h:88:1: warning: symbol 'l' shadows an earlier one
./include/linux/local_lock.h:88:1: originally declared here

after commit

51339d99c0131bc0d16d378e9b05bc498d2967e2 is the first bad commit
commit 51339d99c0131bc0d16d378e9b05bc498d2967e2
Author: Alexei Starovoitov <ast@...nel.org>
Date:   2025-04-02 19:55:14 -0700

   locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type

   Partially revert commit 0aaddfb06882 ("locking/local_lock: Introduce
   localtry_lock_t").  Remove localtry_*() helpers, since localtry_lock()
   name might be misinterpreted as "try lock".

   Introduce local_trylock[_irqsave]() helpers that only work with newly
   introduced local_trylock_t type.  Note that attempt to use
   local_trylock[_irqsave]() with local_lock_t will cause compilation
   failure.

   Usage and behavior in !PREEMPT_RT:

   local_lock_t lock;                     // sizeof(lock) == 0
   local_lock(&lock);                     // preempt disable
   local_lock_irqsave(&lock, ...);        // irq save
   if (local_trylock_irqsave(&lock, ...)) // compilation error

   local_trylock_t lock;                  // sizeof(lock) == 4
   local_lock(&lock);                     // preempt disable, acquired = 1
   local_lock_irqsave(&lock, ...);        // irq save, acquired = 1
   if (local_trylock(&lock))              // if (!acquired) preempt
disable, acquired = 1
   if (local_trylock_irqsave(&lock, ...)) // if (!acquired) irq save,
acquired = 1

   The existing local_lock_*() macros can be used either with local_lock_t or
   local_trylock_t.  With local_trylock_t they set acquired = 1 while
   local_unlock_*() clears it.

   In !PREEMPT_RT local_lock_irqsave(local_lock_t *) disables interrupts to
   protect critical section, but it doesn't prevent NMI, so the fully
   reentrant code cannot use local_lock_irqsave(local_lock_t *) for exclusive
   access.

   The local_lock_irqsave(local_trylock_t *) helper disables interrupts and
   sets acquired=1, so local_trylock_irqsave(local_trylock_t *) from NMI
   attempting to acquire the same lock will return false.

   In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().  Map
   local_trylock_irqsave() to preemptible spin_trylock().  When in hard IRQ
   or NMI return false right away, since 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.

   The local_trylock() without _irqsave can be used to avoid the cost of
   disabling/enabling interrupts by only disabling preemption, so
   local_trylock() in an interrupt attempting to acquire the same lock will
   return false.

   Note there is no need to use local_inc for acquired variable, since it's a
   percpu variable with strict nesting scopes.

   Note that guard(local_lock)(&lock) works only for "local_lock_t lock".

   The patch also makes sure that local_lock_release(l) is called before
   WRITE_ONCE(l->acquired, 0).  Though IRQs are disabled at this point the
   local_trylock() from NMI will succeed and local_lock_acquire(l) will warn.

   Link: https://lkml.kernel.org/r/20250403025514.41186-1-alexei.starovoitov@gmail.com
   Fixes: 0aaddfb06882 ("locking/local_lock: Introduce localtry_lock_t")
   Signed-off-by: Alexei Starovoitov <ast@...nel.org>
   Acked-by: Vlastimil Babka <vbabka@...e.cz>
   Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
   Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>
   Cc: Daniel Borkman <daniel@...earbox.net>
   Cc: Linus Torvalds <torvalds@...uxfoundation.org>
   Cc: Martin KaFai Lau <martin.lau@...nel.org>
   Cc: Michal Hocko <mhocko@...e.com>
   Cc: Peter Zijlstra <peterz@...radead.org>
   Cc: Steven Rostedt <rostedt@...dmis.org>
   Cc: Vlastimil Babka <vbabka@...e.cz>
   Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

include/linux/local_lock.h          |  58 ++--------
include/linux/local_lock_internal.h | 211 +++++++++++++++---------------------
mm/memcontrol.c                     |  39 ++++---
3 files changed, 116 insertions(+), 192 deletions(-)
bisect found first bad commit