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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() {
wlock.inodeLocks = make(map[DevInoStruct]*refCntMutex)
}
// 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 wlock wlockMap
// wlockMap - usage:
// 1) register
// 2) lock ... unlock ...
// 3) unregister
type wlockMap struct {
// opCount counts 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
inodeLocks map[DevInoStruct]*refCntMutex
}
// refCntMutex - mutex with reference count
type refCntMutex struct {
// Write lock for this inode
sync.Mutex
// Reference count
refCnt int
}
// register creates an entry for "ino", or incrementes the reference count
// if the entry already exists.
func (w *wlockMap) register(di DevInoStruct) {
w.Lock()
defer w.Unlock()
r := w.inodeLocks[di]
if r == nil {
r = &refCntMutex{}
w.inodeLocks[di] = r
}
r.refCnt++
}
// unregister decrements the reference count for "di" and deletes the entry if
// the reference count has reached 0.
func (w *wlockMap) unregister(di DevInoStruct) {
w.Lock()
defer w.Unlock()
r := w.inodeLocks[di]
r.refCnt--
if r.refCnt == 0 {
delete(w.inodeLocks, di)
}
}
// lock retrieves the entry for "di" and locks it.
func (w *wlockMap) lock(di DevInoStruct) {
atomic.AddUint64(&w.opCount, 1)
w.Lock()
r := w.inodeLocks[di]
w.Unlock()
// this can take a long time - execute outside the wlockMap lock
r.Lock()
}
// unlock retrieves the entry for "di" and unlocks it.
func (w *wlockMap) unlock(di DevInoStruct) {
w.Lock()
r := w.inodeLocks[di]
w.Unlock()
r.Unlock()
}
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