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([2601:188:c100:5710:315f:57b3:b997:5fca]) by smtp.gmail.com with ESMTPSA id 8926c6da1cb9f-4f2cbdb3b79sm2938719173.1.2025.03.26.11.26.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Mar 2025 11:26:55 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <5f02cf04-74bf-46e5-8104-a62d4aca2bfd@redhat.com> Date: Wed, 26 Mar 2025 14:26:53 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] locking: lockdep: Decrease nr_unused_locks if lock unused in zap_class() To: Boqun Feng , Ingo Molnar , Peter Zijlstra Cc: Will Deacon , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Ingo Molnar References: <20250326180831.510348-1-boqun.feng@gmail.com> Content-Language: en-US In-Reply-To: <20250326180831.510348-1-boqun.feng@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 3/26/25 2:08 PM, Boqun Feng wrote: > Currently, when a lock class is allocated, nr_unused_locks will be > increased by 1, until it gets used: nr_unused_locks will be decreased by > 1 in mark_lock(). However, one scenario is missed: a lock class may be > zapped without even being used once. This could result into a situation > that nr_unused_locks != 0 but no unused lock class is active in the > system, and when `cat /proc/lockdep_stats`, a WARN_ON() will > be triggered in a CONFIG_DEBUG_LOCKDEP=y kernel: > > [...] DEBUG_LOCKS_WARN_ON(debug_atomic_read(nr_unused_locks) != nr_unused) > [...] WARNING: CPU: 41 PID: 1121 at kernel/locking/lockdep_proc.c:283 lockdep_stats_show+0xba9/0xbd0 > > And as a result, lockdep will be disabled after this. > > Therefore, nr_unused_locks needs to be accounted correctly at > zap_class() time. > > Cc: stable@vger.kernel.org > Signee-off-by: Boqun Feng Typo: "Signee-off-by"? Other than that, LGTM Reviewed-by: Waiman Long > --- > kernel/locking/lockdep.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > index b15757e63626..686546d52337 100644 > --- a/kernel/locking/lockdep.c > +++ b/kernel/locking/lockdep.c > @@ -6264,6 +6264,9 @@ static void zap_class(struct pending_free *pf, struct lock_class *class) > hlist_del_rcu(&class->hash_entry); > WRITE_ONCE(class->key, NULL); > WRITE_ONCE(class->name, NULL); > + /* class allocated but not used, -1 in nr_unused_locks */ > + if (class->usage_mask == 0) > + debug_atomic_dec(nr_unused_locks); > nr_lock_classes--; > __clear_bit(class - lock_classes, lock_classes_in_use); > if (class - lock_classes == max_lock_class_idx)