LCOV - code coverage report
Current view: top level - kernel/locking - semaphore.c (source / functions) Hit Total Coverage
Test: combined.info Lines: 31 80 38.8 %
Date: 2022-03-28 13:20:08 Functions: 4 11 36.4 %
Branches: 5 18 27.8 %

           Branch data     Line data    Source code
       1                 :            : // SPDX-License-Identifier: GPL-2.0-only
       2                 :            : /*
       3                 :            :  * Copyright (c) 2008 Intel Corporation
       4                 :            :  * Author: Matthew Wilcox <willy@linux.intel.com>
       5                 :            :  *
       6                 :            :  * This file implements counting semaphores.
       7                 :            :  * A counting semaphore may be acquired 'n' times before sleeping.
       8                 :            :  * See mutex.c for single-acquisition sleeping locks which enforce
       9                 :            :  * rules which allow code to be debugged more easily.
      10                 :            :  */
      11                 :            : 
      12                 :            : /*
      13                 :            :  * Some notes on the implementation:
      14                 :            :  *
      15                 :            :  * The spinlock controls access to the other members of the semaphore.
      16                 :            :  * down_trylock() and up() can be called from interrupt context, so we
      17                 :            :  * have to disable interrupts when taking the lock.  It turns out various
      18                 :            :  * parts of the kernel expect to be able to use down() on a semaphore in
      19                 :            :  * interrupt context when they know it will succeed, so we have to use
      20                 :            :  * irqsave variants for down(), down_interruptible() and down_killable()
      21                 :            :  * too.
      22                 :            :  *
      23                 :            :  * The ->count variable represents how many more tasks can acquire this
      24                 :            :  * semaphore.  If it's zero, there may be tasks waiting on the wait_list.
      25                 :            :  */
      26                 :            : 
      27                 :            : #include <linux/compiler.h>
      28                 :            : #include <linux/kernel.h>
      29                 :            : #include <linux/export.h>
      30                 :            : #include <linux/sched.h>
      31                 :            : #include <linux/sched/debug.h>
      32                 :            : #include <linux/semaphore.h>
      33                 :            : #include <linux/spinlock.h>
      34                 :            : #include <linux/ftrace.h>
      35                 :            : 
      36                 :            : static noinline void __down(struct semaphore *sem);
      37                 :            : static noinline int __down_interruptible(struct semaphore *sem);
      38                 :            : static noinline int __down_killable(struct semaphore *sem);
      39                 :            : static noinline int __down_timeout(struct semaphore *sem, long timeout);
      40                 :            : static noinline void __up(struct semaphore *sem);
      41                 :            : 
      42                 :            : /**
      43                 :            :  * down - acquire the semaphore
      44                 :            :  * @sem: the semaphore to be acquired
      45                 :            :  *
      46                 :            :  * Acquires the semaphore.  If no more tasks are allowed to acquire the
      47                 :            :  * semaphore, calling this function will put the task to sleep until the
      48                 :            :  * semaphore is released.
      49                 :            :  *
      50                 :            :  * Use of this function is deprecated, please use down_interruptible() or
      51                 :            :  * down_killable() instead.
      52                 :            :  */
      53                 :       3048 : void down(struct semaphore *sem)
      54                 :            : {
      55                 :       3048 :         unsigned long flags;
      56                 :            : 
      57                 :       3048 :         raw_spin_lock_irqsave(&sem->lock, flags);
      58         [ +  - ]:       3048 :         if (likely(sem->count > 0))
      59                 :       3048 :                 sem->count--;
      60                 :            :         else
      61                 :          0 :                 __down(sem);
      62                 :       3048 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
      63                 :       3048 : }
      64                 :            : EXPORT_SYMBOL(down);
      65                 :            : 
      66                 :            : /**
      67                 :            :  * down_interruptible - acquire the semaphore unless interrupted
      68                 :            :  * @sem: the semaphore to be acquired
      69                 :            :  *
      70                 :            :  * Attempts to acquire the semaphore.  If no more tasks are allowed to
      71                 :            :  * acquire the semaphore, calling this function will put the task to sleep.
      72                 :            :  * If the sleep is interrupted by a signal, this function will return -EINTR.
      73                 :            :  * If the semaphore is successfully acquired, this function returns 0.
      74                 :            :  */
      75                 :          0 : int down_interruptible(struct semaphore *sem)
      76                 :            : {
      77                 :          0 :         unsigned long flags;
      78                 :          0 :         int result = 0;
      79                 :            : 
      80                 :          0 :         raw_spin_lock_irqsave(&sem->lock, flags);
      81         [ #  # ]:          0 :         if (likely(sem->count > 0))
      82                 :          0 :                 sem->count--;
      83                 :            :         else
      84                 :          0 :                 result = __down_interruptible(sem);
      85                 :          0 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
      86                 :            : 
      87                 :          0 :         return result;
      88                 :            : }
      89                 :            : EXPORT_SYMBOL(down_interruptible);
      90                 :            : 
      91                 :            : /**
      92                 :            :  * down_killable - acquire the semaphore unless killed
      93                 :            :  * @sem: the semaphore to be acquired
      94                 :            :  *
      95                 :            :  * Attempts to acquire the semaphore.  If no more tasks are allowed to
      96                 :            :  * acquire the semaphore, calling this function will put the task to sleep.
      97                 :            :  * If the sleep is interrupted by a fatal signal, this function will return
      98                 :            :  * -EINTR.  If the semaphore is successfully acquired, this function returns
      99                 :            :  * 0.
     100                 :            :  */
     101                 :          0 : int down_killable(struct semaphore *sem)
     102                 :            : {
     103                 :          0 :         unsigned long flags;
     104                 :          0 :         int result = 0;
     105                 :            : 
     106                 :          0 :         raw_spin_lock_irqsave(&sem->lock, flags);
     107         [ #  # ]:          0 :         if (likely(sem->count > 0))
     108                 :          0 :                 sem->count--;
     109                 :            :         else
     110                 :          0 :                 result = __down_killable(sem);
     111                 :          0 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
     112                 :            : 
     113                 :          0 :         return result;
     114                 :            : }
     115                 :            : EXPORT_SYMBOL(down_killable);
     116                 :            : 
     117                 :            : /**
     118                 :            :  * down_trylock - try to acquire the semaphore, without waiting
     119                 :            :  * @sem: the semaphore to be acquired
     120                 :            :  *
     121                 :            :  * Try to acquire the semaphore atomically.  Returns 0 if the semaphore has
     122                 :            :  * been acquired successfully or 1 if it it cannot be acquired.
     123                 :            :  *
     124                 :            :  * NOTE: This return value is inverted from both spin_trylock and
     125                 :            :  * mutex_trylock!  Be careful about this when converting code.
     126                 :            :  *
     127                 :            :  * Unlike mutex_trylock, this function can be used from interrupt context,
     128                 :            :  * and the semaphore can be released by any task or interrupt.
     129                 :            :  */
     130                 :      21815 : int down_trylock(struct semaphore *sem)
     131                 :            : {
     132                 :      21815 :         unsigned long flags;
     133                 :      21815 :         int count;
     134                 :            : 
     135                 :      21815 :         raw_spin_lock_irqsave(&sem->lock, flags);
     136                 :      21815 :         count = sem->count - 1;
     137         [ +  + ]:      21815 :         if (likely(count >= 0))
     138                 :      21785 :                 sem->count = count;
     139                 :      21815 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
     140                 :            : 
     141                 :      21815 :         return (count < 0);
     142                 :            : }
     143                 :            : EXPORT_SYMBOL(down_trylock);
     144                 :            : 
     145                 :            : /**
     146                 :            :  * down_timeout - acquire the semaphore within a specified time
     147                 :            :  * @sem: the semaphore to be acquired
     148                 :            :  * @timeout: how long to wait before failing
     149                 :            :  *
     150                 :            :  * Attempts to acquire the semaphore.  If no more tasks are allowed to
     151                 :            :  * acquire the semaphore, calling this function will put the task to sleep.
     152                 :            :  * If the semaphore is not released within the specified number of jiffies,
     153                 :            :  * this function returns -ETIME.  It returns 0 if the semaphore was acquired.
     154                 :            :  */
     155                 :     361830 : int down_timeout(struct semaphore *sem, long timeout)
     156                 :            : {
     157                 :     361830 :         unsigned long flags;
     158                 :     361830 :         int result = 0;
     159                 :            : 
     160                 :     361830 :         raw_spin_lock_irqsave(&sem->lock, flags);
     161         [ +  - ]:     361830 :         if (likely(sem->count > 0))
     162                 :     361830 :                 sem->count--;
     163                 :            :         else
     164                 :          0 :                 result = __down_timeout(sem, timeout);
     165                 :     361830 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
     166                 :            : 
     167                 :     361830 :         return result;
     168                 :            : }
     169                 :            : EXPORT_SYMBOL(down_timeout);
     170                 :            : 
     171                 :            : /**
     172                 :            :  * up - release the semaphore
     173                 :            :  * @sem: the semaphore to release
     174                 :            :  *
     175                 :            :  * Release the semaphore.  Unlike mutexes, up() may be called from any
     176                 :            :  * context and even by tasks which have never called down().
     177                 :            :  */
     178                 :     386663 : void up(struct semaphore *sem)
     179                 :            : {
     180                 :     386663 :         unsigned long flags;
     181                 :            : 
     182                 :     386663 :         raw_spin_lock_irqsave(&sem->lock, flags);
     183         [ +  - ]:     386663 :         if (likely(list_empty(&sem->wait_list)))
     184                 :     386663 :                 sem->count++;
     185                 :            :         else
     186                 :          0 :                 __up(sem);
     187                 :     386663 :         raw_spin_unlock_irqrestore(&sem->lock, flags);
     188                 :     386663 : }
     189                 :            : EXPORT_SYMBOL(up);
     190                 :            : 
     191                 :            : /* Functions for the contended case */
     192                 :            : 
     193                 :            : struct semaphore_waiter {
     194                 :            :         struct list_head list;
     195                 :            :         struct task_struct *task;
     196                 :            :         bool up;
     197                 :            : };
     198                 :            : 
     199                 :            : /*
     200                 :            :  * Because this function is inlined, the 'state' parameter will be
     201                 :            :  * constant, and thus optimised away by the compiler.  Likewise the
     202                 :            :  * 'timeout' parameter for the cases without timeouts.
     203                 :            :  */
     204                 :          0 : static inline int __sched __down_common(struct semaphore *sem, long state,
     205                 :            :                                                                 long timeout)
     206                 :            : {
     207                 :          0 :         struct semaphore_waiter waiter;
     208                 :            : 
     209                 :          0 :         list_add_tail(&waiter.list, &sem->wait_list);
     210                 :          0 :         waiter.task = current;
     211                 :          0 :         waiter.up = false;
     212                 :            : 
     213                 :          0 :         for (;;) {
     214         [ #  # ]:          0 :                 if (signal_pending_state(state, current))
     215                 :          0 :                         goto interrupted;
     216         [ #  # ]:          0 :                 if (unlikely(timeout <= 0))
     217                 :          0 :                         goto timed_out;
     218                 :          0 :                 __set_current_state(state);
     219                 :          0 :                 raw_spin_unlock_irq(&sem->lock);
     220                 :          0 :                 timeout = schedule_timeout(timeout);
     221                 :          0 :                 raw_spin_lock_irq(&sem->lock);
     222         [ #  # ]:          0 :                 if (waiter.up)
     223                 :            :                         return 0;
     224                 :            :         }
     225                 :            : 
     226                 :            :  timed_out:
     227                 :          0 :         list_del(&waiter.list);
     228                 :          0 :         return -ETIME;
     229                 :            : 
     230                 :            :  interrupted:
     231                 :          0 :         list_del(&waiter.list);
     232                 :          0 :         return -EINTR;
     233                 :            : }
     234                 :            : 
     235                 :          0 : static noinline void __sched __down(struct semaphore *sem)
     236                 :            : {
     237                 :          0 :         __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
     238                 :          0 : }
     239                 :            : 
     240                 :          0 : static noinline int __sched __down_interruptible(struct semaphore *sem)
     241                 :            : {
     242                 :          0 :         return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
     243                 :            : }
     244                 :            : 
     245                 :          0 : static noinline int __sched __down_killable(struct semaphore *sem)
     246                 :            : {
     247                 :          0 :         return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
     248                 :            : }
     249                 :            : 
     250                 :          0 : static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
     251                 :            : {
     252                 :          0 :         return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
     253                 :            : }
     254                 :            : 
     255                 :            : static noinline void __sched __up(struct semaphore *sem)
     256                 :            : {
     257                 :            :         struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
     258                 :            :                                                 struct semaphore_waiter, list);
     259                 :            :         list_del(&waiter->list);
     260                 :            :         waiter->up = true;
     261                 :            :         wake_up_process(waiter->task);
     262                 :            : }

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