blob: 96cec4fc2c5497ddbd2fae2bec59b05bbfde9e20 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
|
package serialize_reads
import (
"log"
"sync"
"time"
"github.com/rfjakob/gocryptfs/internal/tlog"
)
// serializerState is used by the Wait and Done functions
type serializerState struct {
// we get submissions through the "input" channel
input chan *submission
// q = Queue
q []*submission
// wg is used to wait for the read to complete before unblocking the next
wg sync.WaitGroup
}
// Wait places the caller into a queue and blocks
func Wait(offset int64, size int) {
serializer.wait(offset, size)
}
// Done signals that the read operation has finished
func Done() {
serializer.wg.Done()
}
type submission struct {
// "ch" is closed by "eventLoop" once it wants to unblock the caller
ch chan struct{}
// submissions are prioritized by offset (lowest offset gets unblocked first)
offset int64
// size will be used in the future to detect consecutive read requests. These
// can be unblocked immediately.
size int
}
func (sr *serializerState) wait(offset int64, size int) {
ch := make(chan struct{})
sb := &submission{
ch: ch,
offset: offset,
size: size,
}
// Send our submission
sr.input <- sb
// Wait till we get unblocked
<-ch
}
// push returns true if the queue is full after the element has been stored.
// It panics if it did not have space to store the element.
func (sr *serializerState) push(sb *submission) (full bool) {
free := 0
stored := false
for i, v := range sr.q {
if v != nil {
continue
}
if !stored {
sr.q[i] = sb
stored = true
continue
}
free++
}
if !stored {
// This should never happen because eventLoop checks if the queue got full
log.Panic("BUG: unhandled queue overflow")
}
if free == 0 {
return true
}
return false
}
// pop the submission with the lowest offset off the queue
func (sr *serializerState) pop() *submission {
var winner *submission
var winnerIndex int
for i, v := range sr.q {
if v == nil {
continue
}
if winner == nil {
winner = v
winnerIndex = i
continue
}
if v.offset < winner.offset {
winner = v
winnerIndex = i
}
}
if winner == nil {
return nil
}
sr.q[winnerIndex] = nil
return winner
}
func (sr *serializerState) eventLoop() {
sr.input = make(chan *submission)
empty := true
for {
if empty {
// If the queue is empty we block on the channel to conserve CPU
sb := <-sr.input
sr.push(sb)
empty = false
}
select {
case sb := <-sr.input:
full := sr.push(sb)
if full {
// Queue is full, unblock the new request immediately
tlog.Warn.Printf("serialize_reads: queue full, forcing unblock")
sr.unblockOne()
}
case <-time.After(time.Microsecond * 500):
// Looks like we have waited out all concurrent requests.
empty = sr.unblockOne()
}
}
}
// Unblock a submission and wait for completion
func (sr *serializerState) unblockOne() (empty bool) {
winner := sr.pop()
if winner == nil {
return true
}
sr.wg.Add(1)
close(winner.ch)
sr.wg.Wait()
return false
}
var serializer serializerState
// InitSerializer sets up the internal serializer state and starts the event loop.
// Called by fusefrontend.NewFS.
func InitSerializer() {
serializer.input = make(chan *submission)
serializer.q = make([]*submission, 10)
go serializer.eventLoop()
}
|
