The TTY layer now has its own ways to deal with recursive
locking and release-on-sleep for the tty_lock() calls,
meaning that it's safe to replace the Big Kernel Lock
with a subsystem specific Big TTY Mutex (BTM).
This patch for now makes the new behaviour an optional
experimental feature that can be enabled for testing
purposes.
Using a regular mutex here will change the behaviour
when blocked on the BTM from spinning to sleeping,
but that should be visible to the user.
Using the mutex also means that all the BTM is now
covered by lockdep.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
drivers/char/Makefile | 1 +
drivers/char/tty_mutex.c | 100 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/init_task.h | 1 +
include/linux/sched.h | 1 +
include/linux/tty.h | 19 +++++++++
kernel/fork.c | 1 +
lib/Kconfig.debug | 10 +++++
7 files changed, 133 insertions(+), 0 deletions(-)
create mode 100644 drivers/char/tty_mutex.c
diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index f957edf..74ee3fa 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -9,6 +9,7 @@ FONTMAPFILE = cp437.uni
obj-y += mem.o random.o tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o tty_buffer.o tty_port.o
+obj-$(CONFIG_TTY_MUTEX) += tty_mutex.o
obj-$(CONFIG_LEGACY_PTYS) += pty.o
obj-$(CONFIG_UNIX98_PTYS) += pty.o
obj-y += misc.o
diff --git a/drivers/char/tty_mutex.c b/drivers/char/tty_mutex.c
new file mode 100644
index 0000000..51e0852
--- /dev/null
+++ b/drivers/char/tty_mutex.c
@@ -0,0 +1,100 @@
+/*
+ * drivers/char/tty_lock.c
+ */
+#include <linux/tty.h>
+#include <linux/module.h>
+#include <linux/kallsyms.h>
+#include <linux/semaphore.h>
+#include <linux/sched.h>
+
+/*
+ * The 'big kernel semaphore'
+ *
+ * This mutex is taken and released recursively by tty_lock()
+ * and tty_unlock(). It is transparently dropped and reacquired
+ * over schedule(). It is used to protect legacy ...
