lib/kernel_lock.c

   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 #ifdef CONFIG_PREEMPT_BKL
  13 /*
  14  * The 'big kernel semaphore'
  15  *
  16  * This mutex is taken and released recursively by lock_kernel()
  17  * and unlock_kernel().  It is transparently dropped and reacquired
  18  * over schedule().  It is used to protect legacy code that hasn't
  19  * been migrated to a proper locking design yet.
  20  *
  21  * Note: code locked by this semaphore will only be serialized against
  22  * other code using the same locking facility. The code guarantees that
  23  * the task remains on the same CPU.
  24  *
  25  * Don't use in new code.
  26  */
  27 static DECLARE_MUTEX(kernel_sem);
  28
  29 /*
  30  * Re-acquire the kernel semaphore.
  31  *
  32  * This function is called with preemption off.
  33  *
  34  * We are executing in schedule() so the code must be extremely careful
  35  * about recursion, both due to the down() and due to the enabling of
  36  * preemption. schedule() will re-check the preemption flag after
  37  * reacquiring the semaphore.
  38  */
  39 int __lockfunc __reacquire_kernel_lock(void)
  40 {
  41         struct task_struct *task = current;
  42         int saved_lock_depth = task->lock_depth;
  43
  44         BUG_ON(saved_lock_depth < 0);
  45
  46         task->lock_depth = -1;
  47         preempt_enable_no_resched();
  48
  49         down(&kernel_sem);
  50
  51         preempt_disable();
  52         task->lock_depth = saved_lock_depth;
  53
  54         return 0;
  55 }
  56
  57 void __lockfunc __release_kernel_lock(void)
  58 {
  59         up(&kernel_sem);
  60 }
  61
  62 /*
  63  * Getting the big kernel semaphore.
  64  */
  65 void __lockfunc lock_kernel(void)
  66 {
  67         struct task_struct *task = current;
  68         int depth = task->lock_depth + 1;
  69
  70         if (likely(!depth))
  71                 /*
  72                  * No recursion worries - we set up lock_depth _after_
  73                  */
  74                 down(&kernel_sem);
  75
  76         task->lock_depth = depth;
  77 }
  78
  79 void __lockfunc unlock_kernel(void)
  80 {
  81         struct task_struct *task = current;
  82
  83         BUG_ON(task->lock_depth < 0);
  84
  85         if (likely(--task->lock_depth < 0))
  86                 up(&kernel_sem);
  87 }
  88
  89 #else
  90
  91 /*
  92  * The 'big kernel lock'
  93  *
  94  * This spinlock is taken and released recursively by lock_kernel()
  95  * and unlock_kernel().  It is transparently dropped and reacquired
  96  * over schedule().  It is used to protect legacy code that hasn't
  97  * been migrated to a proper locking design yet.
  98  *
  99  * Don't use in new code.
 100  */
 101 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
 102
 103
 104 /*
 105  * Acquire/release the underlying lock from the scheduler.
 106  *
 107  * This is called with preemption disabled, and should
 108  * return an error value if it cannot get the lock and
 109  * TIF_NEED_RESCHED gets set.
 110  *
 111  * If it successfully gets the lock, it should increment
 112  * the preemption count like any spinlock does.
 113  *
 114  * (This works on UP too - _raw_spin_trylock will never
 115  * return false in that case)
 116  */
 117 int __lockfunc __reacquire_kernel_lock(void)
 118 {
 119         while (!_raw_spin_trylock(&kernel_flag)) {
 120                 if (test_thread_flag(TIF_NEED_RESCHED))
 121                         return -EAGAIN;
 122                 cpu_relax();
 123         }
 124         preempt_disable();
 125         return 0;
 126 }
 127
 128 void __lockfunc __release_kernel_lock(void)
 129 {
 130         _raw_spin_unlock(&kernel_flag);
 131         preempt_enable_no_resched();
 132 }
 133
 134 /*
 135  * These are the BKL spinlocks - we try to be polite about preemption.
 136  * If SMP is not on (ie UP preemption), this all goes away because the
 137  * _raw_spin_trylock() will always succeed.
 138  */
 139 #ifdef CONFIG_PREEMPT
 140 static inline void __lock_kernel(void)
 141 {
 142         preempt_disable();
 143         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
 144                 /*
 145                  * If preemption was disabled even before this
 146                  * was called, there's nothing we can be polite
 147                  * about - just spin.
 148                  */
 149                 if (preempt_count() > 1) {
 150                         _raw_spin_lock(&kernel_flag);
 151                         return;
 152                 }
 153
 154                 /*
 155                  * Otherwise, let's wait for the kernel lock
 156                  * with preemption enabled..
 157                  */
 158                 do {
 159                         preempt_enable();
 160                         while (spin_is_locked(&kernel_flag))
 161                                 cpu_relax();
 162                         preempt_disable();
 163                 } while (!_raw_spin_trylock(&kernel_flag));
 164         }
 165 }
 166
 167 #else
 168
 169 /*
 170  * Non-preemption case - just get the spinlock
 171  */
 172 static inline void __lock_kernel(void)
 173 {
 174         _raw_spin_lock(&kernel_flag);
 175 }
 176 #endif
 177
 178 static inline void __unlock_kernel(void)
 179 {
 180         /*
 181          * the BKL is not covered by lockdep, so we open-code the
 182          * unlocking sequence (and thus avoid the dep-chain ops):
 183          */
 184         _raw_spin_unlock(&kernel_flag);
 185         preempt_enable();
 186 }
 187
 188 /*
 189  * Getting the big kernel lock.
 190  *
 191  * This cannot happen asynchronously, so we only need to
 192  * worry about other CPU's.
 193  */
 194 void __lockfunc lock_kernel(void)
 195 {
 196         int depth = current->lock_depth+1;
 197         if (likely(!depth))
 198                 __lock_kernel();
 199         current->lock_depth = depth;
 200 }
 201
 202 void __lockfunc unlock_kernel(void)
 203 {
 204         BUG_ON(current->lock_depth < 0);
 205         if (likely(--current->lock_depth < 0))
 206                 __unlock_kernel();
 207 }
 208
 209 #endif
 210
 211 EXPORT_SYMBOL(lock_kernel);
 212 EXPORT_SYMBOL(unlock_kernel);
 213