locking/local_lock, mm: sparse warnings about shadowed variable

locking/local_lock, mm: sparse warnings about shadowed variable

[Charlemagne Lasse]

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@kernel.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@kernel.org>
   Acked-by: Vlastimil Babka <vbabka@suse.cz>
   Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
   Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
   Cc: Daniel Borkman <daniel@iogearbox.net>
   Cc: Linus Torvalds <torvalds@linuxfoundation.org>
   Cc: Martin KaFai Lau <martin.lau@kernel.org>
   Cc: Michal Hocko <mhocko@suse.com>
   Cc: Peter Zijlstra <peterz@infradead.org>
   Cc: Steven Rostedt <rostedt@goodmis.org>
   Cc: Vlastimil Babka <vbabka@suse.cz>
   Signed-off-by: Andrew Morton <akpm@linux-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

Re: locking/local_lock, mm: sparse warnings about shadowed variable

[Vlastimil Babka]

On 6/11/25 19:37, Charlemagne Lasse wrote:
> 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@kernel.org>
> Date:   2025-04-02 19:55:14 -0700
> 
>    locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type

Looks like __DEFINE_LOCK_GUARD_1() has "_type *l" and __local_lock_acquire()
has "local_lock_t *l;". It can be fixed e.g. like this, although it's
harmless? The _release() part is not necessary, just for symmetry.

diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
index 8d5ac16a9b17..075338f270d0 100644
--- a/include/linux/local_lock_internal.h
+++ b/include/linux/local_lock_internal.h
@@ -97,17 +97,17 @@ do {                                                                \
 #define __local_lock_acquire(lock)                                     \
        do {                                                            \
                local_trylock_t *tl;                                    \
-               local_lock_t *l;                                        \
+               local_lock_t *ll;                                       \
                                                                        \
-               l = (local_lock_t *)this_cpu_ptr(lock);                 \
-               tl = (local_trylock_t *)l;                              \
+               ll = (local_lock_t *)this_cpu_ptr(lock);                \
+               tl = (local_trylock_t *)ll;                             \
                _Generic((lock),                                        \
                        __percpu local_trylock_t *: ({                  \
                                lockdep_assert(tl->acquired == 0);      \
                                WRITE_ONCE(tl->acquired, 1);            \
                        }),                                             \
                        __percpu local_lock_t *: (void)0);              \
-               local_lock_acquire(l);                                  \
+               local_lock_acquire(ll);                                 \
        } while (0)
 
 #define __local_lock(lock)                                     \
@@ -165,11 +165,11 @@ do {                                                              \
 #define __local_lock_release(lock)                                     \
        do {                                                            \
                local_trylock_t *tl;                                    \
-               local_lock_t *l;                                        \
+               local_lock_t *ll;                                       \
                                                                        \
-               l = (local_lock_t *)this_cpu_ptr(lock);                 \
-               tl = (local_trylock_t *)l;                              \
-               local_lock_release(l);                                  \
+               ll = (local_lock_t *)this_cpu_ptr(lock);                \
+               tl = (local_trylock_t *)ll;                             \
+               local_lock_release(ll);                                 \
                _Generic((lock),                                        \
                        __percpu local_trylock_t *: ({                  \
                                lockdep_assert(tl->acquired == 1);      \

Re: locking/local_lock, mm: sparse warnings about shadowed variable

[Alexei Starovoitov]

On Wed, Jun 11, 2025 at 10:57 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 6/11/25 19:37, Charlemagne Lasse wrote:
> > 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@kernel.org>
> > Date:   2025-04-02 19:55:14 -0700
> >
> >    locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type
>
> Looks like __DEFINE_LOCK_GUARD_1() has "_type *l" and __local_lock_acquire()
> has "local_lock_t *l;". It can be fixed e.g. like this, although it's
> harmless? The _release() part is not necessary, just for symmetry.
>
> diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
> index 8d5ac16a9b17..075338f270d0 100644
> --- a/include/linux/local_lock_internal.h
> +++ b/include/linux/local_lock_internal.h
> @@ -97,17 +97,17 @@ do {                                                                \
>  #define __local_lock_acquire(lock)                                     \
>         do {                                                            \
>                 local_trylock_t *tl;                                    \
> -               local_lock_t *l;                                        \
> +               local_lock_t *ll;                                       \

I wouldn't bother messing with the code because of sparse.
Compilers don't warn here.

Re: locking/local_lock, mm: sparse warnings about shadowed variable

[Andrew Morton]

On Wed, 11 Jun 2025 11:20:16 -0700 Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> > diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
> > index 8d5ac16a9b17..075338f270d0 100644
> > --- a/include/linux/local_lock_internal.h
> > +++ b/include/linux/local_lock_internal.h
> > @@ -97,17 +97,17 @@ do {                                                                \
> >  #define __local_lock_acquire(lock)                                     \
> >         do {                                                            \
> >                 local_trylock_t *tl;                                    \
> > -               local_lock_t *l;                                        \
> > +               local_lock_t *ll;                                       \
> 
> I wouldn't bother messing with the code because of sparse.
> Compilers don't warn here.

There's some value in not cluttering up the sparse output in this
fashion.  sparse does often find things which we choose to fix.

Re: locking/local_lock, mm: sparse warnings about shadowed variable

[Charlemagne Lasse]

Am Mi., 11. Juni 2025 um 20:20 Uhr schrieb Alexei Starovoitov
<alexei.starovoitov@gmail.com>:
> I wouldn't bother messing with the code because of sparse.
> Compilers don't warn here.

Actually, they do:

$ make W=2 mm/mlock.o
[...]
In file included from ./include/linux/preempt.h:11,
                from ./include/linux/spinlock.h:56,
                from ./include/linux/wait.h:9,
                from ./include/linux/wait_bit.h:8,
                from ./include/linux/fs.h:7,
                from ./include/linux/mman.h:5,
                from mm/mlock.c:10:
./include/linux/local_lock.h: In function
‘class_local_lock_irqsave_constructor’:
./include/linux/local_lock_internal.h:100:31: warning: declaration of
‘l’ shadows a parameter [-Wshadow]
 100 |                 local_lock_t *l;                                        \
     |                               ^
./include/linux/cleanup.h:394:9: note: in definition of macro
‘__DEFINE_LOCK_GUARD_1’
 394 |         _lock;                                                          \
     |         ^~~~~
./include/linux/local_lock.h:88:1: note: in expansion of macro
‘DEFINE_LOCK_GUARD_1’
  88 | DEFINE_LOCK_GUARD_1(local_lock_irqsave, local_lock_t __percpu,
     | ^~~~~~~~~~~~~~~~~~~
./include/linux/local_lock_internal.h:128:17: note: in expansion of
macro ‘__local_lock_acquire’
 128 |                 __local_lock_acquire(lock);                     \
     |                 ^~~~~~~~~~~~~~~~~~~~
./include/linux/local_lock.h:31:9: note: in expansion of macro
‘__local_lock_irqsave’
  31 |         __local_lock_irqsave(lock, flags)
     |         ^~~~~~~~~~~~~~~~~~~~
./include/linux/local_lock.h:89:21: note: in expansion of macro
‘local_lock_irqsave’
  89 |                     local_lock_irqsave(_T->lock, _T->flags),
     |                     ^~~~~~~~~~~~~~~~~~
./include/linux/cleanup.h:391:68: note: shadowed declaration is here
 391 | static inline class_##_name##_t class_##_name##_constructor(_type *l)   \
./include/linux/cleanup.h:410:1: note: in expansion of macro
‘__DEFINE_LOCK_GUARD_1’
 410 | __DEFINE_LOCK_GUARD_1(_name, _type, _lock)
     | ^~~~~~~~~~~~~~~~~~~~~
./include/linux/local_lock.h:88:1: note: in expansion of macro
‘DEFINE_LOCK_GUARD_1’
  88 | DEFINE_LOCK_GUARD_1


$ gcc --version
gcc (Debian 14.2.0-19) 14.2.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.