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Fri, 11 Jul 2025 15:23:52 +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 9ueLCIgscWjUZQAAD6G6ig (envelope-from ); Fri, 11 Jul 2025 15:23:52 +0000 Message-ID: Date: Fri, 11 Jul 2025 17:23:51 +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: Sebastian Andrzej Siewior Cc: Alexei Starovoitov , bpf@vger.kernel.org, linux-mm@kvack.org, harry.yoo@oracle.com, shakeel.butt@linux.dev, mhocko@suse.com, andrii@kernel.org, memxor@gmail.com, akpm@linux-foundation.org, peterz@infradead.org, rostedt@goodmis.org, hannes@cmpxchg.org 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> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Stat-Signature: zuimtdbscnfgj5ijsawgxtj8687oi56a X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 2A27A1C0008 X-HE-Tag: 1752247433-328767 X-HE-Meta: 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 NpkM/5re +UNPNMN+y38arkVFXZ3+eqGIpwKD/YyPBcgATrWWIAZNwG1efPtofdtiDkg/NtDS5O/DQyIGF4dCC27AcMMGoDSSnHvEFVXoWE1WRhM/twTj0YIXoGCCwr7eZcZ7uumJ54H6HwbdVWTLSNB+9wMlbW8PEibgqYcSxgw2/DsyI5qm6MaMCXdwCrbKuSEv/tJ9i0DQS5vvdHSjngsjlAQAdW5/EJ/nkX2l+fmXlSmBaylthUlhBdkpd6FcgakMDmJiXQEf3e+EUn7eyMnHjGclfQfprmpMm5/NOFx98qKggf8pHh80wo1AB6H7sXi54L56kYwjl4uCeEDx/3vh5vk0BOeeNRuOprf/MyBvKach9WqwUtv6MO1v1NU92gNb2hT/wUJK997hZFleD8BJbOdk3c/LaqaMfHyu7Sx/TKZfsM48zoa//hjS9Khht5DHrzRIxud6en1v5D+C2kE4= 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 7/11/25 17:17, Sebastian Andrzej Siewior wrote: > On 2025-07-11 11:55:22 [+0200], Vlastimil Babka wrote: >> On 7/11/25 09:50, Sebastian Andrzej Siewior wrote: >> > On 2025-07-08 18:53:00 [-0700], Alexei Starovoitov wrote: >> >> From: Alexei Starovoitov >> >> >> >> Introduce local_lock_lockdep_start/end() pair to teach lockdep >> >> about a region of execution where per-cpu local_lock is not taken >> >> and lockdep should consider such local_lock() as "trylock" to >> >> avoid multiple false-positives: >> >> - lockdep doesn't like when the same lock is taken in normal and >> >> in NMI context >> >> - lockdep cannot recognize that local_locks that protect kmalloc >> >> buckets are different local_locks and not taken together >> >> >> >> This pair of lockdep aid is used by slab in the following way: >> >> >> >> if (local_lock_is_locked(&s->cpu_slab->lock)) >> >> 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); >> >> >> >> Where ___slab_alloc() is calling >> >> local_lock_irqsave(&s->cpu_slab->lock, ...) many times, >> >> and all of them will not deadlock since this lock is not taken. >> > >> > So you prefer this instead of using a trylock variant in ___slab_alloc() >> > which would simply return in case the trylock fails? >> >> The code isn't always in a position to "simply return". On !RT I think we >> can at least assume that if we succeeded once, it means we're not a irq/nmi >> interrupting a locked context so we'll succeed the following attempts too. >> On RT IIUC the lock might be taken by someone else, so a trylock might fail >> (even if it should also mean we're in a context that can do a non-try lock). > > There is this parent check. If the parent check "allows" the allocation > then on !RT the trylock should always succeed. So the return "empty > handed" would be there but should not happen kind of thing. > > On RT this is different so local_lock_is_locked() will return false but > the trylock might fail if the lock is acquired by another task. > > But then with this change we do trylock from lockdep's point of view > while in reality we do the full locking including possible context > switch. > > That is why I don't like the part where we trick lockdep. > > If we the parent check we could trylock for !RT and normal lock for RT > what we actual do. > If there is no parent check then we could do "normal lock" on both > sides. So you mean the approach of v1 with local_lock_irqsave_check()? https://lore.kernel.org/bpf/20250501032718.65476-5-alexei.starovoitov@gmail.com/#t > Sebastian