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drasko |
/*
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* Userspace mutex implementation
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*
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* Copyright (C) 2009 Bahadir Bilgehan Balban
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*/
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#include <l4lib/mutex.h>
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#include <l4lib/types.h>
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#include L4LIB_INC_ARCH(syscalls.h)
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#include L4LIB_INC_ARCH(syslib.h)
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/*
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* NOTES:
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*
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* The design is kept as simple as possible.
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*
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* l4_mutex_lock() locks an initialized, mutex.
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* If it contends, it calls the mutex syscall.
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*
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* l4_mutex_unlock() releases an acquired mutex.
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* If there was contention, mutex syscall is called
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* to resolve by the kernel.
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*
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* Internals:
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*
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* (1) The kernel creates a waitqueue for every unique
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* mutex in the system, i.e. every unique physical
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* address that is contended as a mutex. In that respect
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* virtual mutex addresses are translated to physical
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* and checked for match.
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*
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* (2) If a mutex is contended, kernel is called by both the
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* locker and the unlocker (i.e. the lock holder). The syscall
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* results in a rendezvous and both tasks quit the syscall
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* synchronised. A rendezvous is necessary because it is not possible
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* to check lock status and send a WAIT or WAKEUP request to the
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* kernel atomically from userspace. In other words, a WAKEUP call
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* would be lost if it arrived before the unsuccessful lock attempt
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* resulted in a WAIT.
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*
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* (3) The unlocker releases the lock after it returns from the syscall.
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* (4) The locker continuously tries to acquire the lock
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*
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* Issues:
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* - The kernel action is to merely wake up sleepers. If
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* a new thread acquires the lock meanwhile, all those woken
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* up threads would have to sleep again.
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* - All sleepers are woken up (aka thundering herd). This
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* must be done because if a single task is woken up, there
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* is no guarantee that that would in turn wake up others.
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* It might even quit attempting to take the lock.
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* - Whether this is the best design - time will tell.
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*/
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extern int __l4_mutex_lock(void *word);
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extern int __l4_mutex_unlock(void *word);
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void l4_mutex_init(struct l4_mutex *m)
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{
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m->lock = L4_MUTEX_UNLOCKED;
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}
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int l4_mutex_lock(struct l4_mutex *m)
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{
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int err;
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while(__l4_mutex_lock(&m->lock) != L4_MUTEX_SUCCESS) {
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if ((err = l4_mutex_control(&m->lock, L4_MUTEX_LOCK)) < 0) {
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printf("%s: Error: %d\n", __FUNCTION__, err);
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return err;
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}
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}
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return 0;
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}
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int l4_mutex_unlock(struct l4_mutex *m)
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{
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int err, contended;
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if ((contended = __l4_mutex_unlock(m))) {
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if ((err = l4_mutex_control(&m->lock,
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contended | L4_MUTEX_UNLOCK)) < 0) {
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printf("%s: Error: %d\n", __FUNCTION__, err);
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return err;
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}
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}
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return 0;
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}
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