Use fdatasync instead of O_SYNC on storage

Opening the backing files with O_SYNC makes things really slow. So slow
in fact that the modem times out after 10 seconds waiting for the last
EFS sync to go through. I think this takes forever because rmtfs is
doing 512-byte reads and writes.

One option would be to make this bigger. But a better option is to not
use O_SYNC, but explicitly do an fdatasync() after the iovec operation
is complete. This is better because 1) it's way faster, we no longer see
10-12 second delays at rebooto time, and 2) partial syncs of the EFS
file aren't useful anyway.

Use fdatasync() as opposed to fsync() since it's not important for the
metadata to be synced, just the file contents.

Signed-off-by: Evan Green <evangreen86@gmail.com>

rmtfs.c

@@ -220,6 +220,10 @@ static void rmtfs_iovec(int sock, struct qrtr_packet *pkt)
 respond:
 	dbgprintf("[RMTFS] iovec %d, %sforced => (%d:%d)\n", caller_id, force ? "" : "not ",
 							     resp.result.result, resp.result.error);
+
+	if (is_write)
+		storage_sync(rmtfd);
+
 	for (i = 0; i < num_entries; i++) {
 		dbgprintf("[RMTFS]       %s %d:%d 0x%x\n", is_write ? "write" : "read",
 							   entries[i].sector_addr,

rmtfs.h

@@ -34,6 +34,7 @@ int storage_get_error(const struct rmtfd *rmtfd);
 void storage_exit(void);
 ssize_t storage_pread(const struct rmtfd *rmtfd, void *buf, size_t nbyte, off_t offset);
 ssize_t storage_pwrite(struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t offset);
+int storage_sync(struct rmtfd *rmtfd);
 
 int rproc_init(void);
 int rproc_start(void);

storage.c

@@ -122,7 +122,7 @@ found:
 	fspath = alloca(pathlen);
 	snprintf(fspath, pathlen, "%s/%s", storage_dir, file);
 	if (!storage_read_only) {
-		fd = open(fspath, O_RDWR | O_SYNC);
+		fd = open(fspath, O_RDWR);
 		if (fd < 0) {
 			saved_errno = errno;
 			fprintf(stderr, "[storage] failed to open '%s' (requested '%s'): %s\n",
@@ -245,6 +245,14 @@ ssize_t storage_pwrite(struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t
 	return nbyte;
 }
 
+int storage_sync(struct rmtfd *rmtfd)
+{
+	if (storage_read_only)
+		return 0;
+	
+	return fdatasync(rmtfd->fd);
+}
+
 static int storage_populate_shadow_buf(struct rmtfd *rmtfd, const char *file)
 {
 	ssize_t len;