litmus-rt.git - The LITMUS^RT kernel.

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Diffstat (limited to 'arch/arm/kernel/xscale-cp0.c')

0 files changed, 0 insertions, 0 deletions

/* rwsem.c: R/W semaphores: contention handling functions * * Written by David Howells (dhowells@redhat.com). * Derived from arch/i386/kernel/semaphore.c * * Writer lock-stealing by Alex Shi <alex.shi@intel.com> * and Michel Lespinasse <walken@google.com> * * Optimistic spinning by Tim Chen <tim.c.chen@intel.com> * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes. */ #include <linux/rwsem.h> #include <linux/sched.h> #include <linux/init.h> #include <linux/export.h> #include <linux/sched/rt.h> #include <linux/osq_lock.h> #include "rwsem.h" /* * Guide to the rw_semaphore's count field for common values. * (32-bit case illustrated, similar for 64-bit) * * 0x0000000X (1) X readers active or attempting lock, no writer waiting * X = #active_readers + #readers attempting to lock * (X*ACTIVE_BIAS) * * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or * attempting to read lock or write lock. * * 0xffff000X (1) X readers active or attempting lock, with waiters for lock * X = #active readers + # readers attempting lock * (X*ACTIVE_BIAS + WAITING_BIAS) * (2) 1 writer attempting lock, no waiters for lock * X-1 = #active readers + #readers attempting lock * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS) * (3) 1 writer active, no waiters for lock * X-1 = #active readers + #readers attempting lock * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS) * * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock * (WAITING_BIAS + ACTIVE_BIAS) * (2) 1 writer active or attempting lock, no waiters for lock * (ACTIVE_WRITE_BIAS) * * 0xffff0000 (1) There are writers or readers queued but none active * or in the process of attempting lock. * (WAITING_BIAS) * Note: writer can attempt to steal lock for this count by adding * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count * * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue. * (ACTIVE_WRITE_BIAS + WAITING_BIAS) * * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking * the count becomes more than 0 for successful lock acquisition, * i.e. the case where there are only readers or nobody has lock. * (1st and 2nd case above). * * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and * checking the count becomes ACTIVE_WRITE_BIAS for successful lock * acquisition (i.e. nobody else has lock or attempts lock). If * unsuccessful, in rwsem_down_write_failed, we'll check to see if there * are only waiters but none active (5th case above), and attempt to * steal the lock. * */ /* * Initialize an rwsem: */ void __init_rwsem(struct rw_semaphore *sem, const char *name, struct lock_class_key *key) { #ifdef CONFIG_DEBUG_LOCK_ALLOC /* * Make sure we are not reinitializing a held semaphore: */ debug_check_no_locks_freed((void *)sem, sizeof(*sem)); lockdep_init_map(&sem->dep_map, name, key, 0); #endif atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE); raw_spin_lock_init(&sem->wait_lock); INIT_LIST_HEAD(&sem->wait_list); #ifdef CONFIG_RWSEM_SPIN_ON_OWNER sem->owner = NULL; osq_lock_init(&sem->osq); #endif } EXPORT_SYMBOL(__init_rwsem); enum rwsem_waiter_type { RWSEM_WAITING_FOR_WRITE, RWSEM_WAITING_FOR_READ }; struct rwsem_waiter { struct list_head list; struct task_struct *task; enum rwsem_waiter_type type; }; enum rwsem_wake_type { RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */ RWSEM_WAKE_READERS, /* Wake readers only */ RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ }; /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) * - there must be someone on the queue * - the wait_lock must be held by the caller * - tasks are marked for wakeup, the caller must later invoke wake_up_q() * to actually wakeup the blocked task(s) and drop the reference count, * preferably when the wait_lock is released * - woken process blocks are discarded from the list after having task zeroed * - writers are only marked woken if downgrading is false */ static void __rwsem_mark_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type, struct wake_q_head *wake_q) { struct rwsem_waiter *waiter; struct task_struct *tsk; struct list_head *next; long loop, oldcount, woken = 0, adjustment = 0; waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); if (waiter->type == RWSEM_WAITING_FOR_WRITE) { if (wake_type == RWSEM_WAKE_ANY) { /* * Mark writer at the front of the queue for wakeup. * Until the task is actually later awoken later by * the caller, other writers are able to steal it. * Readers, on the other hand, will block as they * will notice the queued writer. */ wake_q_add(wake_q, waiter->task); } return; } /* * Writers might steal the lock before we grant it to the next reader. * We prefer to do the first reader grant before counting readers * so we can bail out early if a writer stole the lock. */ if (wake_type != RWSEM_WAKE_READ_OWNED) { adjustment = RWSEM_ACTIVE_READ_BIAS; try_reader_grant: oldcount = atomic_long_fetch_add(adjustment, &sem->count); if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { /* * If the count is still less than RWSEM_WAITING_BIAS * after removing the adjustment, it is assumed that * a writer has stolen the lock. We have to undo our * reader grant. */ if (atomic_long_add_return(-adjustment, &sem->count) < RWSEM_WAITING_BIAS) return; /* Last active locker left. Retry waking readers. */ goto try_reader_grant; } /* * It is not really necessary to set it to reader-owned here, * but it gives the spinners an early indication that the * readers now have the lock. */ rwsem_set_reader_owned(sem); } /* * Grant an infinite number of read locks to the readers at the front * of the queue. Note we increment the 'active part' of the count by * the number of readers before waking any processes up. */ do { woken++; if (waiter->list.next == &sem->wait_list) break; waiter = list_entry(waiter->list.next, struct rwsem_waiter, list); } while (waiter->type != RWSEM_WAITING_FOR_WRITE); adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; if (waiter->type != RWSEM_WAITING_FOR_WRITE) /* hit end of list above */ adjustment -= RWSEM_WAITING_BIAS; if (adjustment) atomic_long_add(adjustment, &sem->count); next = sem->wait_list.next; loop = woken; do { waiter = list_entry(next, struct rwsem_waiter, list); next = waiter->list.next; tsk = waiter->task; wake_q_add(wake_q, tsk); /* * Ensure that the last operation is setting the reader * waiter to nil such that rwsem_down_read_failed() cannot * race with do_exit() by always holding a reference count * to the task to wakeup. */ smp_store_release(&waiter->task, NULL); } while (--loop); sem->wait_list.next = next; next->prev = &sem->wait_list; } /* * Wait for the read lock to be granted */ __visible struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) { long count, adjustment = -RWSEM_ACTIVE_READ_BIAS; struct rwsem_waiter waiter; struct task_struct *tsk = current; WAKE_Q(wake_q); waiter.task = tsk; waiter.type = RWSEM_WAITING_FOR_READ; raw_spin_lock_irq(&sem->wait_lock); if (list_empty(&sem->wait_list)) adjustment += RWSEM_WAITING_BIAS; list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ count = atomic_long_add_return(adjustment, &sem->count); /* If there are no active locks, wake the front queued process(es). * * If there are no writers and we are first in the queue, * wake our own waiter to join the existing active readers ! */ if (count == RWSEM_WAITING_BIAS || (count > RWSEM_WAITING_BIAS && adjustment != -RWSEM_ACTIVE_READ_BIAS)) __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); raw_spin_unlock_irq(&sem->wait_lock); wake_up_q(&wake_q); /* wait to be given the lock */ while (true) { set_task_state(tsk, TASK_UNINTERRUPTIBLE); if (!waiter.task) break; schedule(); } __set_task_state(tsk, TASK_RUNNING); return sem; } EXPORT_SYMBOL(rwsem_down_read_failed); /* * This function must be called with the sem->wait_lock held to prevent * race conditions between checking the rwsem wait list and setting the * sem->count accordingly. */ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) { /* * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS. */ if (count != RWSEM_WAITING_BIAS) return false; /* * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there * are other tasks on the wait list, we need to add on WAITING_BIAS. */ count = list_is_singular(&sem->wait_list) ? RWSEM_ACTIVE_WRITE_BIAS : RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS; if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) { rwsem_set_owner(sem); return true; } return false; } #ifdef CONFIG_RWSEM_SPIN_ON_OWNER /* * Try to acquire write lock before the writer has been put on wait queue. */ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) { long old, count = atomic_long_read(&sem->count); while (true) { if (!(count == 0 || count == RWSEM_WAITING_BIAS)) return false; old = atomic_long_cmpxchg_acquire(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS); if (old == count) { rwsem_set_owner(sem); return true; } count = old; } } static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) { struct task_struct *owner; bool ret = true; if (need_resched()) return false; rcu_read_lock(); owner = READ_ONCE(sem->owner); if (!rwsem_owner_is_writer(owner)) { /* * Don't spin if the rwsem is readers owned. */ ret = !rwsem_owner_is_reader(owner); goto done; } ret = owner->on_cpu; done: rcu_read_unlock(); return ret; } /* * Return true only if we can still spin on the owner field of the rwsem. */ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) { struct task_struct *owner = READ_ONCE(sem->owner); if (!rwsem_owner_is_writer(owner)) goto out; rcu_read_lock(); while (sem->owner == owner) { /* * Ensure we emit the owner->on_cpu, dereference _after_ * checking sem->owner still matches owner, if that fails, * owner might point to free()d memory, if it still matches, * the rcu_read_lock() ensures the memory stays valid. */ barrier(); /* abort spinning when need_resched or owner is not running */ if (!owner->on_cpu || need_resched()) { rcu_read_unlock(); return false; } cpu_relax_lowlatency(); } rcu_read_unlock(); out: /* * If there is a new owner or the owner is not set, we continue * spinning. */ return !rwsem_owner_is_reader(READ_ONCE(sem->owner)); } static bool rwsem_optimistic_spin(struct rw_semaphore *sem) { bool taken = false; preempt_disable(); /* sem->wait_lock should not be held when doing optimistic spinning */ if (!rwsem_can_spin_on_owner(sem)) goto done; if (!osq_lock(&sem->osq)) goto done; /* * Optimistically spin on the owner field and attempt to acquire the * lock whenever the owner changes. Spinning will be stopped when: * 1) the owning writer isn't running; or * 2) readers own the lock as we can't determine if they are * actively running or not. */ while (rwsem_spin_on_owner(sem)) { /* * Try to acquire the lock */ if (rwsem_try_write_lock_unqueued(sem)) { taken = true; break; } /* * When there's no owner, we might have preempted between the * owner acquiring the lock and setting the owner field. If * we're an RT task that will live-lock because we won't let * the owner complete. */ if (!sem->owner && (need_resched() || rt_task(current))) break; /* * The cpu_relax() call is a compiler barrier which forces * everything in this loop to be re-loaded. We don't need * memory barriers as we'll eventually observe the right * values at the cost of a few extra spins. */ cpu_relax_lowlatency(); } osq_unlock(&sem->osq); done: preempt_enable(); return taken; } /* * Return true if the rwsem has active spinner */ static inline bool rwsem_has_spinner(struct rw_semaphore *sem) { return osq_is_locked(&sem->osq); } #else static bool rwsem_optimistic_spin(struct rw_semaphore *sem) { return false; } static inline bool rwsem_has_spinner(struct rw_semaphore *sem) { return false; } #endif /* * Wait until we successfully acquire the write lock */ static inline struct rw_semaphore * __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) { long count; bool waiting = true; /* any queued threads before us */ struct rwsem_waiter waiter; struct rw_semaphore *ret = sem; WAKE_Q(wake_q); /* undo write bias from down_write operation, stop active locking */ count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count); /* do optimistic spinning and steal lock if possible */ if (rwsem_optimistic_spin(sem)) return sem; /* * Optimistic spinning failed, proceed to the slowpath * and block until we can acquire the sem. */ waiter.task = current; waiter.type = RWSEM_WAITING_FOR_WRITE; raw_spin_lock_irq(&sem->wait_lock); /* account for this before adding a new element to the list */ if (list_empty(&sem->wait_list)) waiting = false; list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ if (waiting) { count = atomic_long_read(&sem->count); /* * If there were already threads queued before us and there are * no active writers, the lock must be read owned; so we try to * wake any read locks that were queued ahead of us. */ if (count > RWSEM_WAITING_BIAS) { WAKE_Q(wake_q); __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q); /* * The wakeup is normally called _after_ the wait_lock * is released, but given that we are proactively waking * readers we can deal with the wake_q overhead as it is * similar to releasing and taking the wait_lock again * for attempting rwsem_try_write_lock(). */ wake_up_q(&wake_q); } } else count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count); /* wait until we successfully acquire the lock */ set_current_state(state); while (true) { if (rwsem_try_write_lock(count, sem)) break; raw_spin_unlock_irq(&sem->wait_lock); /* Block until there are no active lockers. */ do { if (signal_pending_state(state, current)) goto out_nolock;


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