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Peter Anvin" , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] cleanup: Remove NULL check from unconditional guards Message-ID: <20260512124557.GD1889694@noisy.programming.kicks-ass.net> References: <20260512071510.92451-1-d@ilvokhin.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260512071510.92451-1-d@ilvokhin.com> X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 0D91640006 X-Stat-Signature: 54rz4oechtgmrh3tx79qu59p64kzyacs X-Rspam-User: X-HE-Tag: 1778589963-98951 X-HE-Meta: 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 pOZsa/zf ImuQeRirJYyRoBBzFsWm2ijgM7Q8yvd5yAU8DW76v/8GjUN+CR/U4/iNOoUsnpNVG1+0nfjTsdCX4V1ZfQ/LSN8Eebf38j8vS+TtUZzrlcMhKuAqKTLBtsrQ8R03NjpJIZ/3tt9Dd1UAAX/Ze5b0tROenrLWXp6gWET6rKKcmcA9n1o9L2370m5oOsEG3XK4sCsvRYqYLX/eWzKmkvN5zQFMZ2YcJJWzgFDM2YlrSczXmGqU7UWLrY6LgxHeNB762yoKjXljx7BCBKDKVN12zwynZllCP9LeDzwMme/7U068PENYWcd1XnlEvo+WDajvg1FXz Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, May 12, 2026 at 07:15:10AM +0000, Dmitry Ilvokhin wrote: > The unconditional guard destructors check whether the lock pointer is > NULL before unlocking. This check is dead code because unconditional > guards guarantee a non-NULL lock pointer at destructor time. > > DEFINE_GUARD() and DEFINE_LOCK_GUARD_1() both run the lock operation > in the constructor before returning. If the pointer were NULL, the > lock operation (e.g. mutex_lock(NULL)) would crash before the > constructor returns. The destructor never runs with a NULL pointer. > > DEFINE_LOCK_GUARD_0() hardcodes .lock = (void *)1 in the constructor, > so it is never NULL by construction. > > Conditional (_try) variants: DEFINE_GUARD_COND() and > DEFINE_LOCK_GUARD_1_COND() use EXTEND_CLASS_COND(), whose wrapper > destructor returns early when the lock was not acquired, before reaching > the base destructor since 2deccd5c862a ("cleanup: Optimize guards"): > > if (_cond) return; class_##_name##_destructor(_T); > > As compiled by GCC-11 with defconfig on top of the locking/core: > > Total: Before=23889980, After=23833993, chg -0.23% > > Signed-off-by: Dmitry Ilvokhin > --- > Changes in v2: > > - Expand commit message with detailed reasoning, why the proposed > change is correct. > - Rebase on top of locking/core. > > v1: https://lore.kernel.org/all/20260427165037.205337-1-d@ilvokhin.com/ > > See also [1] for relevant discussion. > > [1]: https://lore.kernel.org/all/afCS4d4YccQFtvpi@shell.ilvokhin.com/ > > include/linux/cleanup.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h > index ea95ca4bc11c..1410effa8780 100644 > --- a/include/linux/cleanup.h > +++ b/include/linux/cleanup.h > @@ -397,7 +397,7 @@ static __maybe_unused const bool class_##_name##_is_conditional = _is_cond > __DEFINE_GUARD_LOCK_PTR(_name, _T) > > #define DEFINE_GUARD(_name, _type, _lock, _unlock) \ > - DEFINE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \ > + DEFINE_CLASS(_name, _type, _unlock, ({ _lock; _T; }), _type _T); \ > DEFINE_CLASS_IS_GUARD(_name) > > #define DEFINE_GUARD_COND_4(_name, _ext, _lock, _cond) \ > @@ -491,7 +491,7 @@ typedef struct { \ > static __always_inline void class_##_name##_destructor(class_##_name##_t *_T) \ > __no_context_analysis \ > { \ > - if (_T->lock) { _unlock; } \ > + _unlock; \ > } \ > \ > __DEFINE_GUARD_LOCK_PTR(_name, &_T->lock) What about class_irqdesc_lock_constructor() ? AFAICT __irq_get_desc_lock() can return NULL.