/* * lib/kernel_lock.c * * This is the traditional BKL - big kernel lock. Largely * relegated to obsolescense, but used by various less * important (or lazy) subsystems. */ #include <linux/smp_lock.h> #include <linux/module.h> #include <linux/kallsyms.h> #ifdef CONFIG_PREEMPT_BKL /* * The 'big kernel semaphore' * * This mutex is taken and released recursively by lock_kernel() * and unlock_kernel(). It is transparently dropped and reacquired * over schedule(). It is used to protect legacy code that hasn't * been migrated to a proper locking design yet. * * Note: code locked by this semaphore will only be serialized against * other code using the same locking facility. The code guarantees that * the task remains on the same CPU. * * Don't use in new code. */ static DECLARE_MUTEX(kernel_sem); /* * Re-acquire the kernel semaphore. * * This function is called with preemption off. * * We are executing in schedule() so the code must be extremely careful * about recursion, both due to the down() and due to the enabling of * preemption. schedule() will re-check the preemption flag after * reacquiring the semaphore. */ int __lockfunc __reacquire_kernel_lock(void) { struct task_struct *task = current; int saved_lock_depth = task->lock_depth; BUG_ON(saved_lock_depth < 0); task->lock_depth = -1; preempt_enable_no_resched(); down(&kernel_sem); preempt_disable(); task->lock_depth = saved_lock_depth; return 0; } void __lockfunc __release_kernel_lock(void) { up(&kernel_sem); } /* * Getting the big kernel semaphore. */ void __lockfunc lock_kernel(void) { struct task_struct *task = current; int depth = task->lock_depth + 1; if (likely(!depth)) /* * No recursion worries - we set up lock_depth _after_ */ down(&kernel_sem); task->lock_depth = depth; } void __lockfunc unlock_kernel(void) { struct task_struct *task = current; BUG_ON(task->lock_depth < 0); if (likely(--task->lock_depth < 0)) up(&kernel_sem); } #else /* * The 'big kernel lock' * * This spinlock is taken and released recursively by lock_kernel() * and unlock_kernel(). It is transparently dropped and reacquired * over schedule(). It is used to protect legacy code that hasn't * been migrated to a proper locking design yet. * * Don't use in new code. */ static __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag); /* * Acquire/release the underlying lock from the scheduler. * * This is called with preemption disabled, and should * return an error value if it cannot get the lock and * TIF_NEED_RESCHED gets set. * * If it successfully gets the lock, it should increment * the preemption count like any spinlock does. * * (This works on UP too - _raw_spin_trylock will never * return false in that case) */ int __lockfunc __reacquire_kernel_lock(void) { while (!_raw_spin_trylock(&kernel_flag)) { if (test_thread_flag(TIF_NEED_RESCHED)) return -EAGAIN; cpu_relax(); } preempt_disable(); return 0; } void __lockfunc __release_kernel_lock(void) { _raw_spin_unlock(&kernel_flag); preempt_enable_no_resched(); } /* * These are the BKL spinlocks - we try to be polite about preemption. * If SMP is not on (ie UP preemption), this all goes away because the * _raw_spin_trylock() will always succeed. */ #ifdef CONFIG_PREEMPT static inline void __lock_kernel(void) { preempt_disable(); if (unlikely(!_raw_spin_trylock(&kernel_flag))) { /* * If preemption was disabled even before this * was called, there's nothing we can be polite * about - just spin. */ if (preempt_count() > 1) { _raw_spin_lock(&kernel_flag); return; } /* * Otherwise, let's wait for the kernel lock * with preemption enabled.. */ do { preempt_enable(); while (spin_is_locked(&kernel_flag)) cpu_relax(); preempt_disable(); } while (!_raw_spin_trylock(&kernel_flag)); } } #else /* * Non-preemption case - just get the spinlock */ static inline void __lock_kernel(void) { _raw_spin_lock(&kernel_flag); } #endif static inline void __unlock_kernel(void) { /* * the BKL is not covered by lockdep, so we open-code the * unlocking sequence (and thus avoid the dep-chain ops): */ _raw_spin_unlock(&kernel_flag); preempt_enable(); } /* * Getting the big kernel lock. * * This cannot happen asynchronously, so we only need to * worry about other CPU's. */ void __lockfunc lock_kernel(void) { int depth = current->lock_depth+1; if (likely(!depth)) __lock_kernel(); current->lock_depth = depth; } void __lockfunc unlock_kernel(void) { BUG_ON(current->lock_depth < 0); if (likely(--current->lock_depth < 0)) __unlock_kernel(); } #endif EXPORT_SYMBOL(lock_kernel); EXPORT_SYMBOL(unlock_kernel);