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package fusefrontend
import (
"sync"
"sync/atomic"
"syscall"
)
// DevInoStruct uniquely identifies a backing file through device number and
// inode number.
type DevInoStruct struct {
dev uint64
ino uint64
}
func DevInoFromStat(st *syscall.Stat_t) DevInoStruct {
// Explicit cast to uint64 to prevent build problems on 32-bit platforms
return DevInoStruct{
dev: uint64(st.Dev),
ino: uint64(st.Ino),
}
}
func init() {
openFileMap.entries = make(map[DevInoStruct]*openFileEntryT)
}
// wlock - serializes write accesses to each file (identified by inode number)
// Writing partial blocks means we have to do read-modify-write cycles. We
// really don't want concurrent writes there.
// Concurrent full-block writes could actually be allowed, but are not to
// keep the locking simple.
var openFileMap openFileMapT
// wlockMap - usage:
// 1) register
// 2) lock ... unlock ...
// 3) unregister
type openFileMapT struct {
// opCount counts writeLock.Lock() calls. As every operation that modifies a file should
// call it, this effectively serves as a write-operation counter.
// The variable is accessed without holding any locks so atomic operations
// must be used. It must be the first element of the struct to guarantee
// 64-bit alignment.
opCount uint64
// Protects map access
sync.Mutex
entries map[DevInoStruct]*openFileEntryT
}
type opCountMutex struct {
sync.Mutex
// Points to the opCount variable of the parent openFileMapT
opCount *uint64
}
func (o *opCountMutex) Lock() {
o.Mutex.Lock()
atomic.AddUint64(o.opCount, 1)
}
// refCntMutex - mutex with reference count
type openFileEntryT struct {
// Reference count
refCnt int
// Write lock for this inode
writeLock *opCountMutex
// ID is the file ID in the file header.
ID []byte
IDLock sync.RWMutex
}
// register creates an entry for "ino", or incrementes the reference count
// if the entry already exists.
func (w *openFileMapT) register(di DevInoStruct) *openFileEntryT {
w.Lock()
defer w.Unlock()
r := w.entries[di]
if r == nil {
o := opCountMutex{opCount: &w.opCount}
r = &openFileEntryT{writeLock: &o}
w.entries[di] = r
}
r.refCnt++
return r
}
// unregister decrements the reference count for "di" and deletes the entry if
// the reference count has reached 0.
func (w *openFileMapT) unregister(di DevInoStruct) {
w.Lock()
defer w.Unlock()
r := w.entries[di]
r.refCnt--
if r.refCnt == 0 {
delete(w.entries, di)
}
}
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