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Date: Wed, 14 Dec 2016 15:08:37 -0200
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Fenghua Yu <fenghua.yu@...el.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v3] intelrdt: resctrl: recommend locking for resctrlfs
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@...hat.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);
+}
+
+
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