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([2601:188:c180:4250:ecbe:130d:668d:951d]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4db119f05a0sm57427281cf.43.2025.09.28.08.33.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 28 Sep 2025 08:33:53 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: Date: Sun, 28 Sep 2025 11:33:52 -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 v2] locking/rwsem: add DEBUG_RWSEMS_WARN_ON to warn invalid rwsem To: buckzhang1212@yeah.net, Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org References: <20250927064756.13544-1-buckzhang1212@yeah.net> Content-Language: en-US In-Reply-To: <20250927064756.13544-1-buckzhang1212@yeah.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/27/25 2:47 AM, buckzhang1212@yeah.net wrote: > From: "buck.zhang" > > Here is a kernel exception about rwsem and I can recreate it stably. > First We define a struct contains a rwsem. > Then allocate this struct by kmalloc without calling init_rwsem. > Finally when multiple tasks call use rwsem together,kernel will panic. > This lock is used normally when only one task is using at a time. > the exception reason is that sem->wait_list is an invalid kernel list. > I want to add more warnings when enable CONFIG_DEBUG_RWSEMS > kernel crash log: > Unable to handle kernel NULL pointer dereference at virtual address 0000000 > pc: rwsem_down_write_slowpath+0x428/0xccc > lr: rwsem_down_write_slowpath+0x844/0xccc > sp: ffffffc0870abb00 > x29: ffffffc0870abb60 x28: 0000000000000000 x27: ffffffffffffff05 > ........ > x2: ffffff809d57d830 x1 : 0000000000000000 x0 : 0000000000000000 > Call trace: > rwsem_down_write_slowpath+0x428/0xccc > down_write+0xa8/0x108 > Test case: > struct chip_rwsema { > struct rwsema sem; > }; > static void work_handler1(struct chip_rwsema *csem) > { > down_write(&(csem->sem)); > } > static void work_handler2(struct chip_rwsema *csem) > { > down_write(&(csem->sem)); > } > static void chip_tsem(void) > { > struct chip_rwsema *csem; > csem = kzalloc(sizeof(struct chip_rwsema),GFP_KERNEL); > work_handler1(csem); > ...... > work_handler2(csem); > } > > Signed-off-by: buck.zhang > --- > kernel/locking/rwsem.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c > index 24df4d98f..bfc038573 100644 > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -75,7 +75,17 @@ > list_empty(&(sem)->wait_list) ? "" : "not ")) \ > debug_locks_off(); \ > } while (0) > + > +/* > + * list_invalid - check whether sem->waitlist is invalid > + * @head: the list to test. > + */ > +static inline int rwsem_waitlist_invalid(const struct list_head *head) > +{ > + return (unsigned long) READ_ONCE(head->next) < PAGE_OFFSET; > +} > #else > +# define rwsem_waitlist_invalid(sem) > # define DEBUG_RWSEMS_WARN_ON(c, sem) > #endif > > @@ -1034,6 +1044,9 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat > waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; > waiter.handoff_set = false; > > + /* In case the rwsem is uninitiated, add more warning */ > + DEBUG_RWSEMS_WARN_ON(rwsem_waitlist_invalid(&sem->wait_list), sem); > + > raw_spin_lock_irq(&sem->wait_lock); > if (list_empty(&sem->wait_list)) { > /* > @@ -1128,6 +1141,9 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) > waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; > waiter.handoff_set = false; > > + /* In case the rwsem is uninitiated, add more warning */ > + DEBUG_RWSEMS_WARN_ON(rwsem_waitlist_invalid(&sem->wait_list), sem); > + > raw_spin_lock_irq(&sem->wait_lock); > rwsem_add_waiter(sem, &waiter); > I suppose you are trying to detect call to down_read()/down_write() before the rwsem has been properly initialized. The current way to detect that is to check for sem->magic. Unfortunately, this check may not be run for down_read()/down_write() depending on the whether CONFIG_LOCK_STAT is set or not. Does the below patch work for you for your test case? Cheers, Longman diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 24df4d98f7d2..7e06a6bbdd53 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -1256,6 +1256,7 @@ static __always_inline int __down_read_common(struct rw_semaphore *sem, int stat         int ret = 0;         long count; +       DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);         preempt_disable();         if (!rwsem_read_trylock(sem, &count)) {                 if (IS_ERR(rwsem_down_read_slowpath(sem, count, state))) { @@ -1312,6 +1313,7 @@ static __always_inline int __down_write_common(struct rw_semaphore *sem, int sta  {         int ret = 0; +       DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);         preempt_disable();         if (unlikely(!rwsem_write_trylock(sem))) {                 if (IS_ERR(rwsem_down_write_slowpath(sem, state)))