Re: [PATCH] locking/rwbase: Fix race condition in rwlock_rt fast path

[Waiman Long]

On 9/11/23 08:07, Haodong Wong wrote:
> When the writer in fast path check onwer before sub READER_BIAS,since
> var readers=READER_BIAS, reader thread can jump to critical section to
> access the data that modified by writer which may cause itself panic.
>
> For example we observed below panic in SMP PREEMPT RT:
>
> Call trace:
>
>      try_to_wake_up+0x374/0x5d0
>      default_wake_function+0x3c/0x70
>      autoremove_wake_function+0x3c/0x70
>      __wake_up_common+0x90/0x150
>      __wake_up_common_lock+0x7c/0xc0
>      __wake_up+0x40/0x50
>      ep_poll_callback+0xa0/0x260
>      __wake_up_common+0x90/0x150
>      __wake_up_common_lock+0x7c/0xc0
>      __wake_up_sync_key+0x40/0x50
>      sock_def_readable+0x44/0x90
>      tcp_data_ready+0x3c/0xf0
>      tcp_rcv_established+0x754/0x7f0
>      tcp_v4_do_rcv+0x1f0/0x2e0
>      tcp_v4_rcv+0xb60/0xc80
>      ip_protocol_deliver_rcu+0x44/0x200
>      ip_local_deliver_finish+0x68/0x80
>      ip_local_deliver+0x80/0x130
>      ip_sublist_rcv_finish+0x48/0x70
>      ip_sublist_rcv+0x178/0x1f0
>      ip_list_rcv+0x108/0x1b0
>      __netif_receive_skb_list_core+0x18c/0x220
>      netif_receive_skb_list_internal+0x18c/0x2a0
>      napi_complete_done+0xc4/0x210
>      ether_napi_poll_rx+0xa4/0xe0
>      net_rx_action+0x120/0x430
>      __do_softirq+0x120/0x3b4
>      __local_bh_enable_ip+0xd8/0x140
>      irq_forced_thread_fn+0x84/0xc0
>      irq_thread+0x150/0x260
>      kthread+0x17c/0x1a0
>      ret_from_fork+0x10/0x18
>
> The process is as follows:
> Writer Thread					Reader Thread
>
> ep_poll:					ep_poll_callback:
>    write_lock_irq:				  read_lock_irq:
>
> 	set rtmutex onwer in fast patch
> 							  if reader < 0
>                                                               exit read_lock
>    __remove_wait_queue

After subtracting READER_BIAS, rwbase_write_lock() will wait until the 
reader count is 0 before proceeding. So I don't see an obvious race here 
since the reader count has to be non-zero when that reader enters its 
critical section.


> 						   _wake_up PANIC happen
>
> Signed-off-by: Haodong Wong <haydenw.kernel@xxxxxxxxx>
> ---
>   kernel/locking/rtmutex_common.h | 15 +++++++++++++++
>   kernel/locking/rwbase_rt.c      |  9 +++++++++
>   2 files changed, 24 insertions(+)
>
> diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
> index 1162e07cdaea..d81042657ce2 100644
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -161,6 +161,15 @@ static inline struct task_struct *rt_mutex_owner(struct rt_mutex_base *lock)
>   	return (struct task_struct *) (owner & ~RT_MUTEX_HAS_WAITERS);
>   }
>   
> +static inline struct task_struct *rt_mutex_owner_acquire(struct rt_mutex_base *lock)
> +{
> +	/* Add rt mutex onwer acquire method */
> +	unsigned long owner = (unsigned long) smp_load_acquire(&lock->owner);
> +
> +	return (struct task_struct *) (owner & ~RT_MUTEX_HAS_WAITERS);
> +}
> +
> +
>   /*
>    * Constants for rt mutex functions which have a selectable deadlock
>    * detection.
> @@ -229,6 +238,12 @@ static inline struct task_struct *rt_mutex_owner(struct rt_mutex_base *lock)
>   {
>   	return NULL;
>   }
> +
> +static inline struct task_struct *rt_mutex_owner_acquire(struct rt_mutex_base *lock)
> +{
> +	return NULL;
> +}
> +
>   #endif  /* !CONFIG_RT_MUTEXES */
>   
>   #endif
> diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
> index 25ec0239477c..ca271f57b9c1 100644
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -54,6 +54,15 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
>   {
>   	int r;
>   
> +	/*
> +	 * Reader should check if writer in fast patch in case of they
> +	 * have race condition in the first time.
> +	 */
> +
> +	if ((atomic_read(&lock->readers) == READER_BIAS) &&
> +			rt_mutex_owner_acquire(&lock->rtmutex))
> +		return 0;
> +

Does this patch even compile? There is no variable lock in 
rwbase_read_trylock().

Cheers,
Longman