Add fs.AsyncWithContext and call it in fs/gofer/inodeOperations.Release.

fs/gofer/inodeOperations.Release does some asynchronous work.  Previously it
was calling fs.Async with an anonymous function, which caused the function to
be allocated on the heap.  Because Release is relatively hot, this results in a
lot of small allocations and increased GC pressure, noticeable in perf profiles.

This CL adds a new function, AsyncWithContext, which is just like Async, but
passes a context to the async function.  It avoids the need for an extra
anonymous function in fs/gofer/inodeOperations.Release.  The Async function
itself still requires a single anonymous function.

PiperOrigin-RevId: 233141763
Change-Id: I1dce4a883a7be9a8a5b884db01e654655f16d19c

pkg/sentry/fs/fs.go

@@ -57,6 +57,7 @@ import (
 	"sync"
 
 	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
 )
 
 var (
@@ -87,6 +88,17 @@ func Async(f func()) {
 	}()
 }
 
+// AsyncWithContext is just like Async, except that it calls the asynchronous
+// function with the given context as argument. This function exists to avoid
+// needing to allocate an extra function on the heap in a hot path.
+func AsyncWithContext(ctx context.Context, f func(context.Context)) {
+	workMu.RLock()
+	go func() { // S/R-SAFE: AsyncBarrier must be called.
+		defer workMu.RUnlock() // Ensure RUnlock in case of panic.
+		f(ctx)
+	}()
+}
+
 // AsyncErrorBarrier waits for all outstanding asynchronous work to complete, or
 // the first async error to arrive. Other unfinished async executions will
 // continue in the background. Other past and future async errors are ignored.

pkg/sentry/fs/gofer/inode.go

@@ -352,9 +352,10 @@ func (i *inodeOperations) Release(ctx context.Context) {
 	// Releasing the fileState may make RPCs to the gofer. There is
 	// no need to wait for those to return, so we can do this
 	// asynchronously.
-	fs.Async(func() {
-		i.fileState.Release(ctx)
-	})
+	//
+	// We use AsyncWithContext to avoid needing to allocate an extra
+	// anonymous function on the heap.
+	fs.AsyncWithContext(ctx, i.fileState.Release)
 }
 
 // Mappable implements fs.InodeOperations.Mappable.