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bh=hAeMljy94torG0RE/zR7wOqaRk7RtPGz5NSK370Hp+c=; b=at0T0m9RHqcR8lKW+YjBPxJ72+Px+u8ULawyibBVsS2MHSmRRoZOggCklKypka79izBtFn 5048iHeIcjWv4BkqrQbsuXzw7e2KCDAkaM5DJ1kPnAsSKyKgBzOBqhLF50jLt5kKc74Sr2 2BoZUmO5Jn2S5IfGLuloKxUIboeV8no3lDgXLIcG/xxzB5Uhu0Omt6peqnTbo3SPLYwdWV lljeMP5/kMI3GFjwogWgssIRk0CQZUFF3qFPBGByK66Hacshf5pGw5Qq8MxGmjqk2R6hPF /P5ALTrsPghi3Vdeo47eXpmzqC8agCDdFONBiw0HbfPVdu870k1K0FodExOJlg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1752491200; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hAeMljy94torG0RE/zR7wOqaRk7RtPGz5NSK370Hp+c=; b=Pz9Mr1bhXR8hOu1fTjp+QiWeQRrQEZeJ56sH926MlFsu0EWc0IMNIDFENOfIbk1261Tthj yICWZTJqQf1lxWCw== From: Sebastian Andrzej Siewior To: Alexei Starovoitov Cc: Vlastimil Babka , bpf , linux-mm , Harry Yoo , Shakeel Butt , Michal Hocko , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner Subject: Re: [PATCH v2 3/6] locking/local_lock: Introduce local_lock_lockdep_start/end() Message-ID: <20250714110639.uOaKJEfL@linutronix.de> References: <20250709015303.8107-1-alexei.starovoitov@gmail.com> <20250709015303.8107-4-alexei.starovoitov@gmail.com> <20250711075001.fnlMZfk6@linutronix.de> <1adbee35-6131-49de-835b-2c93aacfdd1e@suse.cz> <20250711151730.rz_TY1Qq@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: E6B3E12000A X-Rspamd-Server: rspam06 X-Stat-Signature: okird4hfiphwbum1a8eew6x5844856a6 X-HE-Tag: 1752491202-387946 X-HE-Meta: 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 YqNHCcjr xd0LVS1GQuqB+zlIH1J+vh3Fuk9pyq1SOq7HB47sJRK8ODM2JAHLP66ztNchpWkfpUoZC1BpYseN4LkvXV/VZUfXKHbinmoFt7LW9NJjNH5feM0sblEkflAmFHXJ0kRCMm373vCk+Y9brSMPA+mR890DgOuXe6gceWmlNWzPzkybyYiUYKjn/4J9hxBJEIEP+D0wQ1q/cK6auyCuLDo6Q0epqTklS8T1H7Gtwznl3I1OyAcA9ErYLLfeVm3vREaQNm7nU/JOEYGUEYFu+fkZAjNaMVg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025-07-11 19:19:26 [-0700], Alexei Starovoitov wrote: > > If there is no parent check then we could do "normal lock" on both > > sides. > > How would ___slab_alloc() know whether there was a parent check or not? > > imo keeping local_lock_irqsave() as-is is cleaner, > since if there is no parent check lockdep will rightfully complain. what about this: diff --git a/mm/slub.c b/mm/slub.c index 7e2ffe1d46c6c..3520d1c25c205 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -3693,6 +3693,34 @@ static inline void *freeze_slab(struct kmem_cache *s, struct slab *slab) return freelist; } +static void local_lock_cpu_slab(struct kmem_cache *s, const gfp_t gfp_flags, + unsigned long *flags) +{ + bool allow_spin = gfpflags_allow_spinning(gfp_flags); + + /* + * ___slab_alloc()'s caller is supposed to check if kmem_cache::kmem_cache_cpu::lock + * can be acquired without a deadlock before invoking the function. + * + * On PREEMPT_RT an invocation is not possible from IRQ-off or preempt + * disabled context. The lock will always be acquired and if needed it + * block and sleep until the lock is available. + * + * On !PREEMPT_RT allocations from any context but NMI are safe. The lock + * is always acquired with disabled interrupts meaning it is always + * possible to it. + * In NMI context it is needed to check if the lock is acquired. If it is not, + * it is safe to acquire it. The trylock semantic is used to tell lockdep + * that we don't spin. The BUG_ON() will not trigger if it is safe to acquire + * the lock. + * + */ + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && !allow_spin) + BUG_ON(!local_trylock_irqsave(&s->cpu_slab->lock, *flags)); + else + local_lock_irqsave(&s->cpu_slab->lock, *flags); +} + /* * Slow path. The lockless freelist is empty or we need to perform * debugging duties. @@ -3765,7 +3793,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, goto deactivate_slab; /* must check again c->slab in case we got preempted and it changed */ - local_lock_irqsave(&s->cpu_slab->lock, flags); + local_lock_cpu_slab(s, gfpflags, &flags); + if (unlikely(slab != c->slab)) { local_unlock_irqrestore(&s->cpu_slab->lock, flags); goto reread_slab; @@ -3803,7 +3832,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, deactivate_slab: - local_lock_irqsave(&s->cpu_slab->lock, flags); + local_lock_cpu_slab(s, gfpflags, &flags); if (slab != c->slab) { local_unlock_irqrestore(&s->cpu_slab->lock, flags); goto reread_slab; @@ -3819,7 +3848,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, #ifdef CONFIG_SLUB_CPU_PARTIAL while (slub_percpu_partial(c)) { - local_lock_irqsave(&s->cpu_slab->lock, flags); + local_lock_cpu_slab(s, gfpflags, &flags); if (unlikely(c->slab)) { local_unlock_irqrestore(&s->cpu_slab->lock, flags); goto reread_slab; @@ -3947,7 +3976,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, retry_load_slab: - local_lock_irqsave(&s->cpu_slab->lock, flags); + local_lock_cpu_slab(s, gfpflags, &flags); if (unlikely(c->slab)) { void *flush_freelist = c->freelist; struct slab *flush_slab = c->slab; @@ -4003,12 +4032,8 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, p = ERR_PTR(-EBUSY); goto out; } - local_lock_lockdep_start(&s->cpu_slab->lock); - p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size); - local_lock_lockdep_end(&s->cpu_slab->lock); - } else { - p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size); } + p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size); out: #ifdef CONFIG_PREEMPT_COUNT slub_put_cpu_ptr(s->cpu_slab); Sebastian