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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 | /* * ARM semaphore implementation, taken from * * i386 semaphore implementation. * * (C) Copyright 1999 Linus Torvalds * * Modified for ARM by Russell King * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. */ #include <linux/module.h> #include <linux/sched.h> #include <linux/errno.h> #include <linux/init.h> #include <asm/semaphore.h> /* * Semaphores are implemented using a two-way counter: * The "count" variable is decremented for each process * that tries to acquire the semaphore, while the "sleeping" * variable is a count of such acquires. * * Notably, the inline "up()" and "down()" functions can * efficiently test if they need to do any extra work (up * needs to do something only if count was negative before * the increment operation. * * "sleeping" and the contention routine ordering is * protected by the semaphore spinlock. * * Note that these functions are only called when there is * contention on the lock, and as such all this is the * "non-critical" part of the whole semaphore business. The * critical part is the inline stuff in <asm/semaphore.h> * where we want to avoid any extra jumps and calls. */ /* * Logic: * - only on a boundary condition do we need to care. When we go * from a negative count to a non-negative, we wake people up. * - when we go from a non-negative count to a negative do we * (a) synchronize with the "sleeper" count and (b) make sure * that we're on the wakeup list before we synchronize so that * we cannot lose wakeup events. */ void __up(struct semaphore *sem) { wake_up(&sem->wait); } static DEFINE_SPINLOCK(semaphore_lock); void __sched __down(struct semaphore * sem) { struct task_struct *tsk = current; DECLARE_WAITQUEUE(wait, tsk); tsk->state = TASK_UNINTERRUPTIBLE; add_wait_queue_exclusive(&sem->wait, &wait); spin_lock_irq(&semaphore_lock); sem->sleepers++; for (;;) { int sleepers = sem->sleepers; /* * Add "everybody else" into it. They aren't * playing, because we own the spinlock. */ if (!atomic_add_negative(sleepers - 1, &sem->count)) { sem->sleepers = 0; break; } sem->sleepers = 1; /* us - see -1 above */ spin_unlock_irq(&semaphore_lock); schedule(); tsk->state = TASK_UNINTERRUPTIBLE; spin_lock_irq(&semaphore_lock); } spin_unlock_irq(&semaphore_lock); remove_wait_queue(&sem->wait, &wait); tsk->state = TASK_RUNNING; wake_up(&sem->wait); } int __sched __down_interruptible(struct semaphore * sem) { int retval = 0; struct task_struct *tsk = current; DECLARE_WAITQUEUE(wait, tsk); tsk->state = TASK_INTERRUPTIBLE; add_wait_queue_exclusive(&sem->wait, &wait); spin_lock_irq(&semaphore_lock); sem->sleepers ++; for (;;) { int sleepers = sem->sleepers; /* * With signals pending, this turns into * the trylock failure case - we won't be * sleeping, and we* can't get the lock as * it has contention. Just correct the count * and exit. */ if (signal_pending(current)) { retval = -EINTR; sem->sleepers = 0; atomic_add(sleepers, &sem->count); break; } /* * Add "everybody else" into it. They aren't * playing, because we own the spinlock. The * "-1" is because we're still hoping to get * the lock. */ if (!atomic_add_negative(sleepers - 1, &sem->count)) { sem->sleepers = 0; break; } sem->sleepers = 1; /* us - see -1 above */ spin_unlock_irq(&semaphore_lock); schedule(); tsk->state = TASK_INTERRUPTIBLE; spin_lock_irq(&semaphore_lock); } spin_unlock_irq(&semaphore_lock); tsk->state = TASK_RUNNING; remove_wait_queue(&sem->wait, &wait); wake_up(&sem->wait); return retval; } /* * Trylock failed - make sure we correct for * having decremented the count. * * We could have done the trylock with a * single "cmpxchg" without failure cases, * but then it wouldn't work on a 386. */ int __down_trylock(struct semaphore * sem) { int sleepers; unsigned long flags; spin_lock_irqsave(&semaphore_lock, flags); sleepers = sem->sleepers + 1; sem->sleepers = 0; /* * Add "everybody else" and us into it. They aren't * playing, because we own the spinlock. */ if (!atomic_add_negative(sleepers, &sem->count)) wake_up(&sem->wait); spin_unlock_irqrestore(&semaphore_lock, flags); return 1; } /* * The semaphore operations have a special calling sequence that * allow us to do a simpler in-line version of them. These routines * need to convert that sequence back into the C sequence when * there is contention on the semaphore. * * ip contains the semaphore pointer on entry. Save the C-clobbered * registers (r0 to r3 and lr), but not ip, as we use it as a return * value in some cases.. * To remain AAPCS compliant (64-bit stack align) we save r4 as well. */ asm(" .section .sched.text,\"ax\",%progbits \n\ .align 5 \n\ .globl __down_failed \n\ __down_failed: \n\ stmfd sp!, {r0 - r4, lr} \n\ mov r0, ip \n\ bl __down \n\ ldmfd sp!, {r0 - r4, pc} \n\ \n\ .align 5 \n\ .globl __down_interruptible_failed \n\ __down_interruptible_failed: \n\ stmfd sp!, {r0 - r4, lr} \n\ mov r0, ip \n\ bl __down_interruptible \n\ mov ip, r0 \n\ ldmfd sp!, {r0 - r4, pc} \n\ \n\ .align 5 \n\ .globl __down_trylock_failed \n\ __down_trylock_failed: \n\ stmfd sp!, {r0 - r4, lr} \n\ mov r0, ip \n\ bl __down_trylock \n\ mov ip, r0 \n\ ldmfd sp!, {r0 - r4, pc} \n\ \n\ .align 5 \n\ .globl __up_wakeup \n\ __up_wakeup: \n\ stmfd sp!, {r0 - r4, lr} \n\ mov r0, ip \n\ bl __up \n\ ldmfd sp!, {r0 - r4, pc} \n\ "); EXPORT_SYMBOL(__down_failed); EXPORT_SYMBOL(__down_interruptible_failed); EXPORT_SYMBOL(__down_trylock_failed); EXPORT_SYMBOL(__up_wakeup); |