2 * Copyright (c) 2005 Jeffrey M. Hsu. All rights reserved.
4 * This code is derived from software contributed to The DragonFly Project
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
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14 * documentation and/or other materials provided with the distribution.
15 * 3. Neither the name of The DragonFly Project nor the names of its
16 * contributors may be used to endorse or promote products derived
17 * from this software without specific, prior written permission.
19 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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22 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
23 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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25 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
26 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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32 * $DragonFly: src/sys/sys/spinlock2.h,v 1.11 2006/06/01 19:02:39 dillon Exp $
35 #ifndef _SYS_SPINLOCK2_H_
36 #define _SYS_SPINLOCK2_H_
40 #error "This file should not be included by userland programs."
45 #include <sys/systm.h>
47 #ifndef _SYS_THREAD2_H_
48 #include <sys/thread2.h>
50 #ifndef _SYS_GLOBALDATA_H_
51 #include <sys/globaldata.h>
53 #ifndef _MACHINE_ATOMIC_H_
54 #include <machine/atomic.h>
56 #ifndef _MACHINE_CPUFUNC_H_
57 #include <machine/cpufunc.h>
61 * SPECIAL NOTE! Obtaining a spinlock does not enter a critical section
62 * or protect against FAST interrupts but it will prevent thread preemption.
63 * Because the spinlock code path is ultra critical, we do not check for
64 * LWKT reschedule requests (due to an interrupt thread not being able to
70 extern int spin_trylock_wr_contested(struct spinlock *mtx, int value);
71 extern void spin_lock_wr_contested(struct spinlock *mtx, int value);
72 extern void spin_lock_rd_contested(struct spinlock *mtx);
79 * Attempt to obtain an exclusive spinlock. Returns FALSE on failure,
80 * TRUE on success. Since the caller assumes that spinlocks must actually
81 * work when using this function, it is only made available to SMP builds.
83 static __inline boolean_t
84 spin_trylock_wr(struct spinlock *mtx)
86 globaldata_t gd = mycpu;
89 ++gd->gd_spinlocks_wr;
90 if ((value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE)) != 0)
91 return (spin_trylock_wr_contested(mtx, value));
98 * Obtain an exclusive spinlock and return. Shortcut the case where the only
99 * cached read lock was from our own cpu (it can just be cleared).
102 spin_lock_wr_quick(globaldata_t gd, struct spinlock *mtx)
108 ++gd->gd_spinlocks_wr;
110 if ((value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE)) != 0) {
111 value &= ~gd->gd_cpumask;
113 spin_lock_wr_contested(mtx, value);
119 spin_lock_wr(struct spinlock *mtx)
121 spin_lock_wr_quick(mycpu, mtx);
127 * Upgrade a shared spinlock to exclusive. Return TRUE if we were
128 * able to upgrade without another exclusive holder getting in before
129 * us, FALSE otherwise.
132 spin_lock_upgrade(struct spinlock *mtx)
134 globaldata_t gd = mycpu;
139 ++gd->gd_spinlocks_wr;
141 value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE);
144 gd->gd_spinlock_rd = NULL;
146 value &= ~gd->gd_cpumask;
148 spin_lock_wr_contested(mtx, value);
149 if (value & SPINLOCK_EXCLUSIVE)
151 XXX regain original shared lock?
160 * Obtain a shared spinlock and return. This is a critical code path.
162 * The vast majority of the overhead is in the cpu_mfence() (5ns vs 1ns for
163 * the entire rest of the procedure). Unfortunately we have to ensure that
164 * spinlock pointer is written out before we check the cpumask to interlock
165 * against an exclusive spinlock that clears the cpumask and then checks
166 * the spinlock pointer.
168 * But what is EXTREMELY important here is that we do not have to perform
169 * a locked bus cycle on the spinlock itself if the shared bit for our cpu
170 * is already found to be set. We only need the mfence, and the mfence is
171 * local to the cpu and never conflicts with other cpu's.
173 * This means that multiple parallel shared acessors (e.g. filedescriptor
174 * table lookups, namecache lookups) run at full speed and incur NO cache
175 * contention at all. It is the difference between 10ns and 40-100ns.
178 spin_lock_rd_quick(globaldata_t gd, struct spinlock *mtx)
180 gd->gd_spinlock_rd = mtx;
183 if ((mtx->lock & gd->gd_cpumask) == 0)
184 spin_lock_rd_contested(mtx);
189 spin_lock_rd(struct spinlock *mtx)
191 spin_lock_rd_quick(mycpu,mtx);
195 * Release an exclusive spinlock. We can just do this passively, only
196 * ensuring that our spinlock count is left intact until the mutex is
200 spin_unlock_wr_quick(globaldata_t gd, struct spinlock *mtx)
205 --gd->gd_spinlocks_wr;
209 spin_unlock_wr(struct spinlock *mtx)
211 spin_unlock_wr_quick(mycpu, mtx);
215 * Release a shared spinlock. We leave the shared bit set in the spinlock
216 * as a cache and simply clear the spinlock pointer for the cpu. This
217 * fast-paths another shared lock later at the cost of an exclusive lock
218 * having to check per-cpu spinlock pointers to determine when there are no
219 * shared holders remaining.
222 spin_unlock_rd_quick(globaldata_t gd, struct spinlock *mtx)
224 KKASSERT(gd->gd_spinlock_rd == mtx);
225 gd->gd_spinlock_rd = NULL;
229 spin_unlock_rd(struct spinlock *mtx)
231 spin_unlock_rd_quick(mycpu, mtx);
235 spin_init(struct spinlock *mtx)
241 spin_uninit(struct spinlock *mtx)
247 #endif /* _SYS_SPINLOCK2_H_ */