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   1 /*
   2  * lib/kernel_lock.c
   3  *
   4  * This is the traditional BKL - big kernel lock. Largely
   5  * relegated to obsolescense, but used by various less
   6  * important (or lazy) subsystems.
   7  */
   8 #include <linux/smp_lock.h>
   9 #include <linux/module.h>
  10 #include <linux/kallsyms.h>
  11
  12 #if defined(CONFIG_PREEMPT) && defined(__smp_processor_id) && \
  13                 defined(CONFIG_DEBUG_PREEMPT)
  14
  15 /*
  16  * Debugging check.
  17  */
  18 unsigned int smp_processor_id(void)
  19 {
  20         unsigned long preempt_count = preempt_count();
  21         int this_cpu = __smp_processor_id();
  22         cpumask_t this_mask;
  23
  24         if (likely(preempt_count))
  25                 goto out;
  26
  27         if (irqs_disabled())
  28                 goto out;
  29
  30         /*
  31          * Kernel threads bound to a single CPU can safely use
  32          * smp_processor_id():
  33          */
  34         this_mask = cpumask_of_cpu(this_cpu);
  35
  36         if (cpus_equal(current->cpus_allowed, this_mask))
  37                 goto out;
  38
  39         /*
  40          * It is valid to assume CPU-locality during early bootup:
  41          */
  42         if (system_state != SYSTEM_RUNNING)
  43                 goto out;
  44
  45         /*
  46          * Avoid recursion:
  47          */
  48         preempt_disable();
  49
  50         if (!printk_ratelimit())
  51                 goto out_enable;
  52
  53         printk(KERN_ERR "BUG: using smp_processor_id() in preemptible [%08x] code: %s/%d\n", preempt_count(), current->comm, current->pid);
  54         print_symbol("caller is %s\n", (long)__builtin_return_address(0));
  55         dump_stack();
  56
  57 out_enable:
  58         preempt_enable_no_resched();
  59 out:
  60         return this_cpu;
  61 }
  62
  63 EXPORT_SYMBOL(smp_processor_id);
  64
  65 #endif /* PREEMPT && __smp_processor_id && DEBUG_PREEMPT */
  66
  67 #ifdef CONFIG_PREEMPT_BKL
  68 /*
  69  * The 'big kernel semaphore'
  70  *
  71  * This mutex is taken and released recursively by lock_kernel()
  72  * and unlock_kernel().  It is transparently dropped and reaquired
  73  * over schedule().  It is used to protect legacy code that hasn't
  74  * been migrated to a proper locking design yet.
  75  *
  76  * Note: code locked by this semaphore will only be serialized against
  77  * other code using the same locking facility. The code guarantees that
  78  * the task remains on the same CPU.
  79  *
  80  * Don't use in new code.
  81  */
  82 static DECLARE_MUTEX(kernel_sem);
  83
  84 /*
  85  * Re-acquire the kernel semaphore.
  86  *
  87  * This function is called with preemption off.
  88  *
  89  * We are executing in schedule() so the code must be extremely careful
  90  * about recursion, both due to the down() and due to the enabling of
  91  * preemption. schedule() will re-check the preemption flag after
  92  * reacquiring the semaphore.
  93  */
  94 int __lockfunc __reacquire_kernel_lock(void)
  95 {
  96         struct task_struct *task = current;
  97         int saved_lock_depth = task->lock_depth;
  98
  99         BUG_ON(saved_lock_depth < 0);
 100
 101         task->lock_depth = -1;
 102         preempt_enable_no_resched();
 103
 104         down(&kernel_sem);
 105
 106         preempt_disable();
 107         task->lock_depth = saved_lock_depth;
 108
 109         return 0;
 110 }
 111
 112 void __lockfunc __release_kernel_lock(void)
 113 {
 114         up(&kernel_sem);
 115 }
 116
 117 /*
 118  * Getting the big kernel semaphore.
 119  */
 120 void __lockfunc lock_kernel(void)
 121 {
 122         struct task_struct *task = current;
 123         int depth = task->lock_depth + 1;
 124
 125         if (likely(!depth))
 126                 /*
 127                  * No recursion worries - we set up lock_depth _after_
 128                  */
 129                 down(&kernel_sem);
 130
 131         task->lock_depth = depth;
 132 }
 133
 134 void __lockfunc unlock_kernel(void)
 135 {
 136         struct task_struct *task = current;
 137
 138         BUG_ON(task->lock_depth < 0);
 139
 140         if (likely(--task->lock_depth < 0))
 141                 up(&kernel_sem);
 142 }
 143
 144 #else
 145
 146 /*
 147  * The 'big kernel lock'
 148  *
 149  * This spinlock is taken and released recursively by lock_kernel()
 150  * and unlock_kernel().  It is transparently dropped and reaquired
 151  * over schedule().  It is used to protect legacy code that hasn't
 152  * been migrated to a proper locking design yet.
 153  *
 154  * Don't use in new code.
 155  */
 156 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
 157
 158
 159 /*
 160  * Acquire/release the underlying lock from the scheduler.
 161  *
 162  * This is called with preemption disabled, and should
 163  * return an error value if it cannot get the lock and
 164  * TIF_NEED_RESCHED gets set.
 165  *
 166  * If it successfully gets the lock, it should increment
 167  * the preemption count like any spinlock does.
 168  *
 169  * (This works on UP too - _raw_spin_trylock will never
 170  * return false in that case)
 171  */
 172 int __lockfunc __reacquire_kernel_lock(void)
 173 {
 174         while (!_raw_spin_trylock(&kernel_flag)) {
 175                 if (test_thread_flag(TIF_NEED_RESCHED))
 176                         return -EAGAIN;
 177                 cpu_relax();
 178         }
 179         preempt_disable();
 180         return 0;
 181 }
 182
 183 void __lockfunc __release_kernel_lock(void)
 184 {
 185         _raw_spin_unlock(&kernel_flag);
 186         preempt_enable_no_resched();
 187 }
 188
 189 /*
 190  * These are the BKL spinlocks - we try to be polite about preemption.
 191  * If SMP is not on (ie UP preemption), this all goes away because the
 192  * _raw_spin_trylock() will always succeed.
 193  */
 194 #ifdef CONFIG_PREEMPT
 195 static inline void __lock_kernel(void)
 196 {
 197         preempt_disable();
 198         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
 199                 /*
 200                  * If preemption was disabled even before this
 201                  * was called, there's nothing we can be polite
 202                  * about - just spin.
 203                  */
 204                 if (preempt_count() > 1) {
 205                         _raw_spin_lock(&kernel_flag);
 206                         return;
 207                 }
 208
 209                 /*
 210                  * Otherwise, let's wait for the kernel lock
 211                  * with preemption enabled..
 212                  */
 213                 do {
 214                         preempt_enable();
 215                         while (spin_is_locked(&kernel_flag))
 216                                 cpu_relax();
 217                         preempt_disable();
 218                 } while (!_raw_spin_trylock(&kernel_flag));
 219         }
 220 }
 221
 222 #else
 223
 224 /*
 225  * Non-preemption case - just get the spinlock
 226  */
 227 static inline void __lock_kernel(void)
 228 {
 229         _raw_spin_lock(&kernel_flag);
 230 }
 231 #endif
 232
 233 static inline void __unlock_kernel(void)
 234 {
 235         _raw_spin_unlock(&kernel_flag);
 236         preempt_enable();
 237 }
 238
 239 /*
 240  * Getting the big kernel lock.
 241  *
 242  * This cannot happen asynchronously, so we only need to
 243  * worry about other CPU's.
 244  */
 245 void __lockfunc lock_kernel(void)
 246 {
 247         int depth = current->lock_depth+1;
 248         if (likely(!depth))
 249                 __lock_kernel();
 250         current->lock_depth = depth;
 251 }
 252
 253 void __lockfunc unlock_kernel(void)
 254 {
 255         BUG_ON(current->lock_depth < 0);
 256         if (likely(--current->lock_depth < 0))
 257                 __unlock_kernel();
 258 }
 259
 260 #endif
 261
 262 EXPORT_SYMBOL(lock_kernel);
 263 EXPORT_SYMBOL(unlock_kernel);
 264