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Tue, 15 Jul 2025 06:56:21 +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 Mhi3H5X7dWjsNgAAD6G6ig (envelope-from ); Tue, 15 Jul 2025 06:56:21 +0000 Message-ID: <6835614d-c316-4ecf-ae2b-52687a66ae7c@suse.cz> Date: Tue, 15 Jul 2025 08:56:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/6] locking/local_lock: Introduce local_lock_lockdep_start/end() To: Alexei Starovoitov Cc: Sebastian Andrzej Siewior , 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> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: E6EFB20011 X-Stat-Signature: tqp3gsshf3fzgpa3p8b9fe7t3m1iix3y X-HE-Tag: 1752562584-331610 X-HE-Meta: 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 4VDr3R3r idZ2OjPnNRUucB7pIOT1tCJRUGWBV+gyK5BFxNdNPEr43TqCRakEXX6U5vfJhg7CwCcm2WXBFSrkeQumTsd8I3/UOzBJcDlzALT9106sDxau2v8BQVBOavPJ8Px2UJ3yKQQHOfbsplwYcjLdUUztJ6wtxYJkKaf/C0xWO5rG5e4mgEXW0tl3f3kz3D/P8SaG92epA7CUnODjyvkyp8PGBmeC89NC07LgKeMoUNqG5/TtVrHKm3rGRRR202SkCMJZuc4Aw6G4OnXsrmGe6fUrCB8vDZZkFkzXIbsCucCCYSOG0pko7SvLtH2Bzw1g0B4rbg+vTY2JNTgy5ZihePY/MYzUjw50GShZeZiU23h0xTXILIvd/O1DACiGBrEIsh+TgRvPt00VeELRqTBe5UphgkkU901ZmMyTnj2k9JRUq1n13DhI= 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 20:46, Alexei Starovoitov wrote: > On Mon, Jul 14, 2025 at 11:33 AM Vlastimil Babka wrote: >> >> 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. > > It's not theoretical. Such slab re-entrance can happen with > a tracepoint: > slab -> some tracepoint -> bpf -> slab > > I simulate it with a stress test: > +extern void (*debug_callback)(void); > +#define local_unlock_irqrestore(lock, flags) \ > + do { \ > + if (debug_callback) debug_callback(); \ > + __local_unlock_irqrestore(lock, flags); \ > + } while (0) > > and debug_callback() calls kmalloc_nolock(random_size) without any bpf > to simplify testing. > >> > [ 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? > > yes, but they are truly different local_locks of different > kmalloc buckets, and local_lock_is_locked() is working. > > See in the splat: > >> > [ 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 > > the addresses of the locks are different and they're different > kmalloc buckets, but lockdep cannot understand this without > explicit local_lock_lockdep_start(). > The same thing I'm trying to explain in the commit log. Thanks for the explanation and sorry for being so dense. Maybe lockdep's lock classes can be used here somehow instead of having to teach lockdep completely new tricks, but I don't know enough about those to know for sure.