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authorJakob Unterwurzacher2016-01-24 13:08:08 +0100
committerJakob Unterwurzacher2016-01-24 13:08:08 +0100
commit2f32114bd356c2124d75fb1879ff230fc1f0ca1b (patch)
tree07c09e6926882bbb859e0c5999860a90833a10ee /pathfs_frontend/write_lock.go
parentdac9f71089b1f20493b29edcec848ab188944990 (diff)
Add per-inode write mutex
At the moment, FUSE writes to a single file are serialized by the kernel. However, it is unclear if this is guaranteed behaviour or may change in the future. This patch adds our own per-inode write lock to rule out races regardless of kernel behavoir.
Diffstat (limited to 'pathfs_frontend/write_lock.go')
-rw-r--r--pathfs_frontend/write_lock.go66
1 files changed, 66 insertions, 0 deletions
diff --git a/pathfs_frontend/write_lock.go b/pathfs_frontend/write_lock.go
new file mode 100644
index 0000000..0704eb6
--- /dev/null
+++ b/pathfs_frontend/write_lock.go
@@ -0,0 +1,66 @@
+package pathfs_frontend
+
+import (
+ "sync"
+)
+
+func init() {
+ wlock.m = make(map[uint64]*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 {
+ mapMutex sync.RWMutex
+ m map[uint64]*refCntMutex
+}
+
+func (w *wlockMap) register(ino uint64) {
+ w.mapMutex.Lock()
+ r := w.m[ino]
+ if r == nil {
+ r = &refCntMutex{}
+ w.m[ino] = r
+ }
+ r.refCnt++ // this must happen inside the mapMutex lock
+ w.mapMutex.Unlock()
+}
+
+func (w *wlockMap) unregister(ino uint64) {
+ w.mapMutex.Lock()
+ r := w.m[ino]
+ r.refCnt--
+ if r.refCnt == 0 {
+ delete(w.m, ino)
+ }
+ w.mapMutex.Unlock()
+}
+
+func (w *wlockMap) lock(ino uint64) {
+ w.mapMutex.RLock()
+ r := w.m[ino]
+ w.mapMutex.RUnlock()
+ r.Lock() // this can take a long time - execute outside the mapMutex lock
+}
+
+func (w *wlockMap) unlock(ino uint64) {
+ w.mapMutex.RLock()
+ r := w.m[ino]
+ w.mapMutex.RUnlock()
+ r.Unlock()
+}
+
+// refCntMutex - mutex with reference count
+type refCntMutex struct {
+ sync.Mutex
+ refCnt int
+}