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113 lines
3.5 KiB
C
113 lines
3.5 KiB
C
/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
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*
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* Written by David Howells (dhowells@redhat.com).
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*
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* Derived from asm-x86/semaphore.h
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*
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*
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* The MSW of the count is the negated number of active writers and waiting
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* lockers, and the LSW is the total number of active locks
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*
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* The lock count is initialized to 0 (no active and no waiting lockers).
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*
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* When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
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* uncontended lock. This can be determined because XADD returns the old value.
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* Readers increment by 1 and see a positive value when uncontended, negative
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* if there are writers (and maybe) readers waiting (in which case it goes to
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* sleep).
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*
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* The value of WAITING_BIAS supports up to 32766 waiting processes. This can
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* be extended to 65534 by manually checking the whole MSW rather than relying
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* on the S flag.
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*
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* The value of ACTIVE_BIAS supports up to 65535 active processes.
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*
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* This should be totally fair - if anything is waiting, a process that wants a
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* lock will go to the back of the queue. When the currently active lock is
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* released, if there's a writer at the front of the queue, then that and only
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* that will be woken up; if there's a bunch of consequtive readers at the
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* front, then they'll all be woken up, but no other readers will be.
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*/
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#ifndef _ASM_X86_RWSEM_H
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#define _ASM_X86_RWSEM_H
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#ifndef _LINUX_RWSEM_H
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#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
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#endif
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#ifdef __KERNEL__
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#include <asm/asm.h>
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#define FASTCALL __attribute__ ((fastcall)) __attribute__ ((dllimport))
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void FASTCALL InitRwsem(struct rw_semaphore *sem)__asm__("InitRwsem");
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void FASTCALL DownRead(struct rw_semaphore *sem)__asm__("DownRead");
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void FASTCALL DownWrite(struct rw_semaphore *sem)__asm__("DownWrite");
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void FASTCALL UpRead(struct rw_semaphore *sem)__asm__("UpRead");
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void FASTCALL UpWrite(struct rw_semaphore *sem)__asm__("UpWrite");
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static inline void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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InitRwsem(sem);
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}
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/*
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* lock for reading
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*/
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static inline void down_read(struct rw_semaphore *sem)
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{
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DownRead(sem);
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}
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static inline void down_write(struct rw_semaphore *sem)
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{
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DownWrite(sem);
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}
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/*
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* unlock after reading
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*/
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static inline void up_read(struct rw_semaphore *sem)
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{
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UpRead(sem);
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}
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/*
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* unlock after writing
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*/
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static inline void up_write(struct rw_semaphore *sem)
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{
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UpWrite(sem);
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}
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#define RWSEM_UNLOCKED_VALUE 0x00000000L
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#define RWSEM_ACTIVE_BIAS 0x00000001L
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#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
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#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
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#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
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static inline int down_read_trylock(struct rw_semaphore *sem)
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{
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long result, tmp;
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asm volatile("# beginning __down_read_trylock\n\t"
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" mov %0,%1\n\t"
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"1:\n\t"
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" mov %1,%2\n\t"
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" add %3,%2\n\t"
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" jle 2f\n\t"
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LOCK_PREFIX " cmpxchg %2,%0\n\t"
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" jnz 1b\n\t"
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"2:\n\t"
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"# ending __down_read_trylock\n\t"
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: "+m" (sem->count), "=&a" (result), "=&r" (tmp)
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: "i" (RWSEM_ACTIVE_READ_BIAS)
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: "memory", "cc");
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return result >= 0 ? 1 : 0;
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}
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#endif /* __KERNEL__ */
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#endif /* _ASM_X86_RWSEM_H */
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