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git.proxmox.com Git - mirror_ubuntu-bionic-kernel.git/blob - kernel/locking/osq_lock.c
1 #include <linux/percpu.h>
2 #include <linux/sched.h>
3 #include <linux/osq_lock.h>
6 * An MCS like lock especially tailored for optimistic spinning for sleeping
7 * lock implementations (mutex, rwsem, etc).
9 * Using a single mcs node per CPU is safe because sleeping locks should not be
10 * called from interrupt context and we have preemption disabled while
13 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node
, osq_node
);
16 * We use the value 0 to represent "no CPU", thus the encoded value
17 * will be the CPU number incremented by 1.
19 static inline int encode_cpu(int cpu_nr
)
24 static inline struct optimistic_spin_node
*decode_cpu(int encoded_cpu_val
)
26 int cpu_nr
= encoded_cpu_val
- 1;
28 return per_cpu_ptr(&osq_node
, cpu_nr
);
32 * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
33 * Can return NULL in case we were the last queued and we updated @lock instead.
35 static inline struct optimistic_spin_node
*
36 osq_wait_next(struct optimistic_spin_queue
*lock
,
37 struct optimistic_spin_node
*node
,
38 struct optimistic_spin_node
*prev
)
40 struct optimistic_spin_node
*next
= NULL
;
41 int curr
= encode_cpu(smp_processor_id());
45 * If there is a prev node in queue, then the 'old' value will be
46 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
47 * we're currently last in queue, then the queue will then become empty.
49 old
= prev
? prev
->cpu
: OSQ_UNLOCKED_VAL
;
52 if (atomic_read(&lock
->tail
) == curr
&&
53 atomic_cmpxchg_acquire(&lock
->tail
, curr
, old
) == curr
) {
55 * We were the last queued, we moved @lock back. @prev
56 * will now observe @lock and will complete its
63 * We must xchg() the @node->next value, because if we were to
64 * leave it in, a concurrent unlock()/unqueue() from
65 * @node->next might complete Step-A and think its @prev is
68 * If the concurrent unlock()/unqueue() wins the race, we'll
69 * wait for either @lock to point to us, through its Step-B, or
70 * wait for a new @node->next from its Step-C.
73 next
= xchg(&node
->next
, NULL
);
78 cpu_relax_lowlatency();
84 bool osq_lock(struct optimistic_spin_queue
*lock
)
86 struct optimistic_spin_node
*node
= this_cpu_ptr(&osq_node
);
87 struct optimistic_spin_node
*prev
, *next
;
88 int curr
= encode_cpu(smp_processor_id());
96 * We need both ACQUIRE (pairs with corresponding RELEASE in
97 * unlock() uncontended, or fastpath) and RELEASE (to publish
98 * the node fields we just initialised) semantics when updating
101 old
= atomic_xchg(&lock
->tail
, curr
);
102 if (old
== OSQ_UNLOCKED_VAL
)
105 prev
= decode_cpu(old
);
107 WRITE_ONCE(prev
->next
, node
);
110 * Normally @prev is untouchable after the above store; because at that
111 * moment unlock can proceed and wipe the node element from stack.
113 * However, since our nodes are static per-cpu storage, we're
114 * guaranteed their existence -- this allows us to apply
115 * cmpxchg in an attempt to undo our queueing.
118 while (!READ_ONCE(node
->locked
)) {
120 * If we need to reschedule bail... so we can block.
125 cpu_relax_lowlatency();
131 * Step - A -- stabilize @prev
133 * Undo our @prev->next assignment; this will make @prev's
134 * unlock()/unqueue() wait for a next pointer since @lock points to us
139 if (prev
->next
== node
&&
140 cmpxchg(&prev
->next
, node
, NULL
) == node
)
144 * We can only fail the cmpxchg() racing against an unlock(),
145 * in which case we should observe @node->locked becomming
148 if (smp_load_acquire(&node
->locked
))
151 cpu_relax_lowlatency();
154 * Or we race against a concurrent unqueue()'s step-B, in which
155 * case its step-C will write us a new @node->prev pointer.
157 prev
= READ_ONCE(node
->prev
);
161 * Step - B -- stabilize @next
163 * Similar to unlock(), wait for @node->next or move @lock from @node
167 next
= osq_wait_next(lock
, node
, prev
);
174 * @prev is stable because its still waiting for a new @prev->next
175 * pointer, @next is stable because our @node->next pointer is NULL and
176 * it will wait in Step-A.
179 WRITE_ONCE(next
->prev
, prev
);
180 WRITE_ONCE(prev
->next
, next
);
185 void osq_unlock(struct optimistic_spin_queue
*lock
)
187 struct optimistic_spin_node
*node
, *next
;
188 int curr
= encode_cpu(smp_processor_id());
191 * Fast path for the uncontended case.
193 if (likely(atomic_cmpxchg_release(&lock
->tail
, curr
,
194 OSQ_UNLOCKED_VAL
) == curr
))
198 * Second most likely case.
200 node
= this_cpu_ptr(&osq_node
);
201 next
= xchg(&node
->next
, NULL
);
203 WRITE_ONCE(next
->locked
, 1);
207 next
= osq_wait_next(lock
, node
, NULL
);
209 WRITE_ONCE(next
->locked
, 1);