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authorPeter Hurley <peter@hurleysoftware.com>2013-04-16 06:15:50 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-05-20 12:30:32 -0700
commit4898e640caf03fdbaf2122d5a33949bf3e4a5b34 (patch)
tree180f6665f2cc00c5595d7e8329ca05e46cbf67f4 /include/linux/tty_ldisc.h
parent50539dd4f88e8a689a38c94337768fd7ff3fd326 (diff)
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tty: Add timed, writer-prioritized rw semaphore
The semantics of a rw semaphore are almost ideally suited for tty line discipline lifetime management; multiple active threads obtain "references" (read locks) while performing i/o to prevent the loss or change of the current line discipline (write lock). Unfortunately, the existing rw_semaphore is ill-suited in other ways; 1) TIOCSETD ioctl (change line discipline) expects to return an error if the line discipline cannot be exclusively locked within 5 secs. Lock wait timeouts are not supported by rwsem. 2) A tty hangup is expected to halt and scrap pending i/o, so exclusive locking must be prioritized. Writer priority is not supported by rwsem. Add ld_semaphore which implements these requirements in a semantically similar way to rw_semaphore. Writer priority is handled by separate wait lists for readers and writers. Pending write waits are priortized before existing read waits and prevent further read locks. Wait timeouts are trivially added, but obviously change the lock semantics as lock attempts can fail (but only due to timeout). This implementation incorporates the write-lock stealing work of Michel Lespinasse <walken@google.com>. Cc: Michel Lespinasse <walken@google.com> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include/linux/tty_ldisc.h')
-rw-r--r--include/linux/tty_ldisc.h46
1 files changed, 46 insertions, 0 deletions
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index 58390c7..7b24bbd 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -110,6 +110,52 @@
#include <linux/wait.h>
#include <linux/wait.h>
+
+/*
+ * the semaphore definition
+ */
+struct ld_semaphore {
+ long count;
+ raw_spinlock_t wait_lock;
+ unsigned int wait_readers;
+ struct list_head read_wait;
+ struct list_head write_wait;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+extern void __init_ldsem(struct ld_semaphore *sem, const char *name,
+ struct lock_class_key *key);
+
+#define init_ldsem(sem) \
+do { \
+ static struct lock_class_key __key; \
+ \
+ __init_ldsem((sem), #sem, &__key); \
+} while (0)
+
+
+extern int ldsem_down_read(struct ld_semaphore *sem, long timeout);
+extern int ldsem_down_read_trylock(struct ld_semaphore *sem);
+extern int ldsem_down_write(struct ld_semaphore *sem, long timeout);
+extern int ldsem_down_write_trylock(struct ld_semaphore *sem);
+extern void ldsem_up_read(struct ld_semaphore *sem);
+extern void ldsem_up_write(struct ld_semaphore *sem);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+extern int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass,
+ long timeout);
+extern int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
+ long timeout);
+#else
+# define ldsem_down_read_nested(sem, subclass, timeout) \
+ ldsem_down_read(sem, timeout)
+# define ldsem_down_write_nested(sem, subclass, timeout) \
+ ldsem_down_write(sem, timeout)
+#endif
+
+
struct tty_ldisc_ops {
int magic;
char *name;
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