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Mon, 14 Jul 2025 18:33:41 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id zyJLNIVNdWjWbAAAD6G6ig (envelope-from ); Mon, 14 Jul 2025 18:33:41 +0000 Message-ID: Date: Mon, 14 Jul 2025 20:33:41 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/6] locking/local_lock: Introduce local_lock_lockdep_start/end() Content-Language: en-US To: Alexei Starovoitov , Sebastian Andrzej Siewior Cc: bpf , linux-mm , Harry Yoo , Shakeel Butt , Michal Hocko , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner 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> <20250714110639.uOaKJEfL@linutronix.de> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 9FD9540008 X-Stat-Signature: e7ug1f8m9ge1mjkfty81578kdfxfo34k X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1752518023-488976 X-HE-Meta: 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 E5J9w+CZ 0SsvQrnNuonxT+rJGAvgNoXm5JXv4Nlxp8rMkz/ZXszNdWgjNSM1h4RXMnj1dXZXCtD4HIedRQhM98m5x/bD6oA5WZg== 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 7/14/25 19:52, Alexei Starovoitov wrote: > On Mon, Jul 14, 2025 at 4:06 AM Sebastian Andrzej Siewior > wrote: >> >> 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); >> +} > > the patch misses these two: > > diff --git a/mm/slub.c b/mm/slub.c > index 36779519b02c..2f30b85fbf68 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -3260,7 +3260,7 @@ static void put_cpu_partial(struct kmem_cache > *s, struct slab *slab, int drain) > unsigned long flags; > int slabs = 0; > > - local_lock_irqsave(&s->cpu_slab->lock, flags); > + local_lock_cpu_slab(s, 0, &flags); > > oldslab = this_cpu_read(s->cpu_slab->partial); > > @@ -4889,8 +4889,9 @@ static __always_inline void do_slab_free(struct > kmem_cache *s, > goto redo; > } > } else { > + long flags; > /* Update the free list under the local lock */ > - local_lock(&s->cpu_slab->lock); > + local_lock_cpu_slab(s, 0, &flags); > c = this_cpu_ptr(s->cpu_slab); > if (unlikely(slab != c->slab)) { > local_unlock(&s->cpu_slab->lock); > > I realized that the latter one was missing local_lock_lockdep_start/end() > in my patch as well, but that's secondary. > > So with above it works on !RT, > but on RT lockdep complains as I explained earlier. > > With yours and above hunks applied here is full lockdep splat: > > [ 39.819636] ============================================ > [ 39.819638] WARNING: possible recursive locking detected > [ 39.819641] 6.16.0-rc5-00342-gc8aca7837440-dirty #54 Tainted: G O > [ 39.819645] -------------------------------------------- > [ 39.819646] page_alloc_kthr/2306 is trying to acquire lock: > [ 39.819650] ff110001f5cbea88 ((&c->lock)){+.+.}-{3:3}, at: > ___slab_alloc+0xb7/0xec0 > [ 39.819667] > [ 39.819667] but task is already holding lock: > [ 39.819668] ff110001f5cbfe88 ((&c->lock)){+.+.}-{3:3}, at: > ___slab_alloc+0xb7/0xec0 > [ 39.819677] > [ 39.819677] other info that might help us debug this: > [ 39.819678] Possible unsafe locking scenario: > [ 39.819678] > [ 39.819679] CPU0 > [ 39.819680] ---- > [ 39.819681] lock((&c->lock)); > [ 39.819684] lock((&c->lock)); > [ 39.819687] > [ 39.819687] *** DEADLOCK *** > [ 39.819687] > [ 39.819687] May be due to missing lock nesting notation > [ 39.819687] > [ 39.819689] 2 locks held by page_alloc_kthr/2306: > [ 39.819691] #0: ff110001f5cbfe88 ((&c->lock)){+.+.}-{3:3}, at: > ___slab_alloc+0xb7/0xec0 > [ 39.819700] #1: ffffffff8588f3a0 (rcu_read_lock){....}-{1:3}, at: > rt_spin_lock+0x197/0x250 > [ 39.819710] > [ 39.819710] stack backtrace: > [ 39.819714] CPU: 1 UID: 0 PID: 2306 Comm: page_alloc_kthr Tainted: > G O 6.16.0-rc5-00342-gc8aca7837440-dirty #54 > PREEMPT_RT > [ 39.819721] Tainted: [O]=OOT_MODULE > [ 39.819723] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), > BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014 > [ 39.819726] Call Trace: > [ 39.819729] > [ 39.819734] dump_stack_lvl+0x5b/0x80 > [ 39.819740] print_deadlock_bug.cold+0xbd/0xca > [ 39.819747] __lock_acquire+0x12ad/0x2590 > [ 39.819753] ? __lock_acquire+0x42b/0x2590 > [ 39.819758] lock_acquire+0x133/0x2d0 > [ 39.819763] ? ___slab_alloc+0xb7/0xec0 > [ 39.819769] ? try_to_take_rt_mutex+0x624/0xfc0 > [ 39.819773] ? __lock_acquire+0x42b/0x2590 > [ 39.819778] rt_spin_lock+0x6f/0x250 But why are we here in ___slab_alloc, trying to take the lock... > [ 39.819783] ? ___slab_alloc+0xb7/0xec0 > [ 39.819788] ? rtlock_slowlock_locked+0x5c60/0x5c60 > [ 39.819792] ? rtlock_slowlock_locked+0xc3/0x5c60 > [ 39.819798] ___slab_alloc+0xb7/0xec0 > [ 39.819803] ? __lock_acquire+0x42b/0x2590 > [ 39.819809] ? my_debug_callback+0x20e/0x390 [bpf_testmod] > [ 39.819826] ? __lock_acquire+0x42b/0x2590 > [ 39.819830] ? rt_read_unlock+0x2f0/0x2f0 > [ 39.819835] ? my_debug_callback+0x20e/0x390 [bpf_testmod] > [ 39.819844] ? kmalloc_nolock_noprof+0x15a/0x430 > [ 39.819849] kmalloc_nolock_noprof+0x15a/0x430 When in patch 6/6 __slab_alloc() we should have bailed out via if (unlikely(!gfpflags_allow_spinning(gfpflags))) { + if (local_lock_is_locked(&s->cpu_slab->lock)) { + /* + * EBUSY is an internal signal to kmalloc_nolock() to + * retry a different bucket. It's not propagated + * to the caller. + */ + p = ERR_PTR(-EBUSY); + goto out; + } So it doesn't seem to me as a lack of lockdep tricking, but we reached something we should not have because the avoidance based on local_lock_is_locked() above didn't work properly? At least if I read the splat and backtrace properly, it doesn't seem to suggest a theoretical scenario but that we really tried to lock something we already had locked. > [ 39.819857] my_debug_callback+0x20e/0x390 [bpf_testmod] What exactly did you instrument here? > [ 39.819867] ? page_alloc_kthread+0x320/0x320 [bpf_testmod] > [ 39.819875] ? lock_is_held_type+0x85/0xe0 > [ 39.819881] ___slab_alloc+0x256/0xec0 And here we took the lock originally? > [ 39.819898] ? lock_acquire+0x133/0x2d0 > [ 39.819927] ? __kmalloc_cache_noprof+0xd6/0x3b0 > [ 39.819932] __kmalloc_cache_noprof+0xd6/0x3b0 > > As I said earlier lockdep _has_ to be tricked. > We cannot unconditionally call local_lock_irqsave() on RT. > lockdep doesn't understand per-cpu local_lock. > And it doesn't understand this "if !locked_by_current_task -> go and lock" > concept. > lockdep has to be taught about safe lock region (call it tricking > lockdep, but it has to be an external signal to lockdep).