[PATCH v3] intelrdt: resctrl: recommend locking for resctrlfs

[Marcelo Tosatti <mtosatti@redhat.com>]

There is a locking problem between different applications
reading/writing to resctrlfs directory at the same time
(read the patch below for details).

Suggest a standard locking scheme for applications to use.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
index d918d26..ec3a58b 100644
--- a/Documentation/x86/intel_rdt_ui.txt
+++ b/Documentation/x86/intel_rdt_ui.txt
@@ -212,3 +212,118 @@ Finally we move core 4-7 over to the new group and make sure that the
 kernel and the tasks running there get 50% of the cache.
 
 # echo C0 > p0/cpus
+
+4) Locking between applications
+
+Certain operations on the resctrl filesystem, composed of
+read / writes to multiple files, must be atomic.
+
+As an example, the allocation of an exclusive reservation
+of L3 cache involves:
+
+        1. read list of cbmmasks for each directory
+        2. find a contiguous set of bits in the global CBM bitmask
+           that is clear in any of the directory cbmmasks
+        3. create a new directory
+        4. set the bits found in step 2 to the new directory "schemata"
+           file
+
+If two applications attempting to allocate space race with each other
+(if two processes execute the steps above in a interlocked fashion),
+they can end up using the same bits of CBMMASK, which renders the
+reservations non-exclusive but shared.
+
+To coordinate atomic operations on resctrl and avoid the problem
+above, the following locking procedure is recommended:
+
+WRITE LOCK:
+
+ A) Take flock(LOCK_EX) on /sys/fs/resctrl
+ B) read/write the directory structure.
+ C) funlock
+
+READ LOCK:
+
+ A) Take flock(LOCK_SH) on /sys/fs/resctrl
+ B) If success read the directory structure.
+ C) funlock
+
+Example with bash:
+
+# Atomically read directory structure
+$ flock -s /sys/fs/resctrl/ find /sys/fs/resctrl
+
+# Read directory contents and create new subdirectory
+
+$ cat create-dir.sh
+find /sys/fs/resctrl/ > output.txt
+mask = function-of(output.txt)
+mkdir /sys/fs/resctrl/newres/
+echo mask > /sys/fs/resctrl/newres/schemata
+
+$ flock /sys/fs/resctrl/ ./create-dir.sh
+
+Example with C:
+
+/*
+ * Example code do take advisory locks
+ * before accessing resctrl filesystem
+ */
+#include <sys/file.h>
+#include <stdlib.h>
+
+void resctrl_take_shared_lock(int fd)
+{
+	int ret;
+
+	/* take shared lock on resctrl filesystem */
+	ret = flock(fd, LOCK_SH);
+	if (ret) {
+		perror("flock");
+		exit(-1);
+	}
+}
+
+void resctrl_take_exclusive_lock(int fd)
+{
+	int ret;
+
+	/* release lock on resctrl filesystem */
+	ret = flock(fd, LOCK_EX);
+	if (ret) {
+		perror("flock");
+		exit(-1);
+	}
+}
+
+void resctrl_release_lock(int fd)
+{
+	int ret;
+
+	/* take shared lock on resctrl filesystem */
+	ret = flock(fd, LOCK_UN);
+	if (ret) {
+		perror("flock");
+		exit(-1);
+	}
+}
+
+void main(void)
+{
+	int fd, ret;
+
+	fd = open("/sys/fs/resctrl", O_DIRECTORY);
+	if (fd == -1) {
+		perror("open");
+		exit(-1);
+	}
+	resctrl_take_shared_lock(fd);
+	/* code to read directory contents */
+	resctrl_release_lock(fd);
+
+	resctrl_take_exclusive_lock(fd);
+	/* code to read and write directory contents */
+	resctrl_release_lock(fd);
+}
+
+