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

[Haodong Wong <haydenw.kernel@gmail.com>]

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

						   _wake_up PANIC happen

Signed-off-by: Haodong Wong <haydenw.kernel@gmail.com>
---
 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;
+
 	/*
 	 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
 	 * set.
-- 
2.25.1