[PATCH] 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>

--- Documentation/x86/intel_rdt_ui.txt.orig	2016-11-30 13:40:33.080233101 -0200
+++ Documentation/x86/intel_rdt_ui.txt	2016-11-30 13:45:01.253703259 -0200
@@ -212,3 +212,30 @@ Finally we move core 4-7 over to the new
 kernel and the tasks running there get 50% of the cache.
 
 # echo C0 > p0/cpus
+
+4) Locking between applications
+
+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 attempt 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 creation of reservations on resctrl and avoid the problem
+above, the following locking procedure is recommended:
+
+A) open /var/lock/resctrl/fs.lock with O_CREAT|O_EXCL.
+B) if success, write pid of program accessing the directory
+   structure to this file.
+C) read/write the directory structure.
+D) remove file.
+