Add DOUBLEFADVISE compile time option

Makefile

 default: all
 all: cachestats cachedel nocache.so
 
+GCC = gcc $(CFLAGS)
 %.c: Makefile
 cachestats: cachestats.c
-	gcc -Wall -o cachestats cachestats.c
+	$(GCC) -Wall -o cachestats cachestats.c
 cachedel: cachedel.c
-	gcc -Wall -o cachedel cachedel.c
+	$(GCC) -Wall -o cachedel cachedel.c
 nocache.o: nocache.c
-	gcc -Wall -fPIC -c -o nocache.o nocache.c
+	$(GCC) -Wall -fPIC -c -o nocache.o nocache.c
 fcntl_helpers.o: fcntl_helpers.c
-	gcc -Wall -fPIC -c -o fcntl_helpers.o fcntl_helpers.c
+	$(GCC) -Wall -fPIC -c -o fcntl_helpers.o fcntl_helpers.c
 nocache.so: nocache.o fcntl_helpers.o
-	gcc -Wall -pthread -shared -Wl,-soname,nocache.so -o nocache.so nocache.o fcntl_helpers.o -ldl
+	$(GCC) -Wall -pthread -shared -Wl,-soname,nocache.so -o nocache.so nocache.o fcntl_helpers.o -ldl
 
 install: all
 	install -m 0644 nocache.so /usr/local/lib

README

@@ -90,6 +90,13 @@ the file descriptor is closed, this may not happen if they are left
 open when the application exits, although the destructor tries to do
 that.
 
+There are timing issues to consider, as well. If you consider `nocache
+cat <file>`, in most (all?) cases the cache will not be restored. For
+discussion and possible solutions see <http://lwn.net/Articles/480930/>.
+My experience showed that in many cases you could "fix" this by doing
+the `posix_fadvise` call *twice*. If you want this behaviour, compile
+the library using `make CFLAGS=-DDOUBLEFADVISE`.
+
 
 Acknowledgements
 ----------------

fcntl_helpers.c

@@ -7,6 +7,9 @@
 
 int fadv_dontneed(int fd, off_t offset, off_t len)
 {
+#ifdef DOUBLEFADVISE
+        posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
+#endif
         return posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
 }