Make tmpfs size accounting lock-free by using atomics.

This is likely to be a speed-up in the common case when contention between
threads is low. In such situations, generally atomic operations are faster.

PiperOrigin-RevId: 493731992

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -971,21 +971,22 @@ func (fs *filesystem) accountPagesPartial(pagesInc uint64) uint64 {
 		return pagesInc
 	}
 
-	// Need to acquire fs.pagesUsedMu for fs.pagesUsed.
-	fs.pagesUsedMu.Lock()
-	defer fs.pagesUsedMu.Unlock()
-	if fs.maxSizeInPages <= fs.pagesUsed {
-		return 0
-	}
+	for {
+		pagesUsed := fs.pagesUsed.Load()
+		if fs.maxSizeInPages <= pagesUsed {
+			return 0
+		}
 
-	pagesFree := fs.maxSizeInPages - fs.pagesUsed
-	if pagesFree < pagesInc {
-		fs.pagesUsed += pagesFree
-		return pagesFree
-	}
+		pagesFree := fs.maxSizeInPages - pagesUsed
+		toInc := pagesInc
+		if pagesFree < pagesInc {
+			toInc = pagesFree
+		}
 
-	fs.pagesUsed += pagesInc
-	return pagesInc
+		if fs.pagesUsed.CompareAndSwap(pagesUsed, pagesUsed+toInc) {
+			return toInc
+		}
+	}
 }
 
 // accountPages increases the pagesUsed in filesystem struct if tmpfs
@@ -996,20 +997,21 @@ func (fs *filesystem) accountPages(pagesInc uint64) bool {
 		return true // No accounting needed.
 	}
 
-	// Need to acquire fs.pagesUsedMu for fs.pagesUsed.
-	fs.pagesUsedMu.Lock()
-	defer fs.pagesUsedMu.Unlock()
-	if fs.maxSizeInPages <= fs.pagesUsed {
-		return false
-	}
+	for {
+		pagesUsed := fs.pagesUsed.Load()
+		if fs.maxSizeInPages <= pagesUsed {
+			return false
+		}
 
-	pagesFree := fs.maxSizeInPages - fs.pagesUsed
-	if pagesFree < pagesInc {
-		return false
-	}
+		pagesFree := fs.maxSizeInPages - pagesUsed
+		if pagesFree < pagesInc {
+			return false
+		}
 
-	fs.pagesUsed += pagesInc
-	return true
+		if fs.pagesUsed.CompareAndSwap(pagesUsed, pagesUsed+pagesInc) {
+			return true
+		}
+	}
 }
 
 // unaccountPages decreases the pagesUsed in filesystem struct if tmpfs
@@ -1018,11 +1020,14 @@ func (fs *filesystem) unaccountPages(pagesDec uint64) {
 	if fs.maxSizeInPages == 0 || pagesDec == 0 {
 		return
 	}
-	// Need to acquire fs.pagesUsedMu for fs.pagesUsed.
-	fs.pagesUsedMu.Lock()
-	defer fs.pagesUsedMu.Unlock()
-	if fs.pagesUsed < pagesDec {
-		panic(fmt.Sprintf("Deallocating more pages than allocated: fs.pagesUsed = %d, pagesDec = %d", fs.pagesUsed, pagesDec))
+
+	for {
+		pagesUsed := fs.pagesUsed.Load()
+		if pagesUsed < pagesDec {
+			panic(fmt.Sprintf("Deallocating more pages than allocated: fs.pagesUsed = %d, pagesDec = %d", pagesUsed, pagesDec))
+		}
+		if fs.pagesUsed.CompareAndSwap(pagesUsed, pagesUsed-pagesDec) {
+			break
+		}
 	}
-	fs.pagesUsed -= pagesDec
 }

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -92,10 +92,8 @@ type filesystem struct {
 	// This field is immutable.
 	maxSizeInPages uint64
 
-	// pagesUsed is the pages used out of the tmpfs size.
-	// pagesUsed is protected by pagesUsedMu.
-	pagesUsedMu pagesUsedMutex `state:"nosave"`
-	pagesUsed   uint64
+	// pagesUsed is the number of pages used by this filesystem.
+	pagesUsed atomicbitops.Uint64
 }
 
 // Name implements vfs.FilesystemType.Name.
@@ -321,10 +319,9 @@ func (fs *filesystem) statFS() linux.Statfs {
 	if fs.maxSizeInPages > 0 {
 		// If size is set for tmpfs return set values.
 		st.Blocks = fs.maxSizeInPages
-		fs.pagesUsedMu.Lock()
-		defer fs.pagesUsedMu.Unlock()
-		st.BlocksFree = fs.maxSizeInPages - fs.pagesUsed
-		st.BlocksAvailable = fs.maxSizeInPages - fs.pagesUsed
+		pagesUsed := fs.pagesUsed.Load()
+		st.BlocksFree = fs.maxSizeInPages - pagesUsed
+		st.BlocksAvailable = fs.maxSizeInPages - pagesUsed
 		return st
 	}
 	// In Linux, if tmpfs is mounted with no size option,