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Subject[PATCH v5 28/44] tty: Remove ldsem recursion support
Date
Read lock recursion is no longer required for ldisc references;
remove mechanism.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldsem.c | 83 +++++------------------------------------------
include/linux/tty_ldisc.h | 2 --
2 files changed, 8 insertions(+), 77 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index c162295..a60d7e3 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -3,28 +3,14 @@
*
* The ldisc semaphore is semantically a rw_semaphore but which enforces
* an alternate policy, namely:
- * 1) Recursive read locking is allowed
- * 2) Supports lock wait timeouts
- * 3) Write waiter has priority, even if lock is already read owned, except:
- * 4) Write waiter does not prevent recursive locking
- * 5) Downgrading is not supported (because of #3 & #4 above)
+ * 1) Supports lock wait timeouts
+ * 2) Write waiter has priority
+ * 3) Downgrading is not supported
*
* Implementation notes:
* 1) Upper half of semaphore count is a wait count (differs from rwsem
* in that rwsem normalizes the upper half to the wait bias)
* 2) Lacks overflow checking
- * 3) Read recursion is tracked with a bitmap indexed by hashed 'current'
- * This approach results in some false positives; ie, a non-recursive
- * read lock may be granted while a write lock is waited.
- * However, this approach does not produce false-negatives
- * (ie. not granting a read lock to a recursive attempt) which might
- * deadlock.
- * Testing the bitmap need not be atomic wrt. setting the bitmap
- * (as the 'current' thread cannot contend with itself); however,
- * since the bitmap is cleared when write lock is granted.
- * Note: increasing the bitmap size reduces the probability of false
- * positives, and thus the probability of granting a non-recursive
- * read lock with writer(s) waiting.
*
* The generic counting was copied and modified from include/asm-generic/rwsem.h
* by Paul Mackerras <paulus@samba.org>.
@@ -53,12 +39,12 @@
# ifdef CONFIG_PROVE_LOCKING
# define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 2, NULL, i)
# define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 2, n, i)
-# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 2, 2, NULL, i)
+# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 2, NULL, i)
# define lockdep_release(l, n, i) __rel(l, n, i)
# else
# define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
# define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
-# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 2, 1, NULL, i)
+# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
# define lockdep_release(l, n, i) __rel(l, n, i)
# endif
#else
@@ -107,26 +93,6 @@ static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
}


-static inline unsigned long __hash_current(void)
-{
- return (unsigned long)current % TASK_MAP_BITS;
-}
-
-static inline void ldsem_clear_task_map(struct ld_semaphore *sem)
-{
- bitmap_zero(sem->task_map, TASK_MAP_BITS);
-}
-
-static inline void ldsem_update_task_map(struct ld_semaphore *sem)
-{
- __set_bit(__hash_current(), sem->task_map);
-}
-
-static inline int ldsem_lock_recursion(struct ld_semaphore *sem)
-{
- return test_bit(__hash_current(), sem->task_map);
-}
-
/*
* Initialize an ldsem:
*/
@@ -144,7 +110,6 @@ void __init_ldsem(struct ld_semaphore *sem, const char *name,
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->read_wait);
INIT_LIST_HEAD(&sem->write_wait);
- ldsem_clear_task_map(sem);
}

static void __ldsem_wake_readers(struct ld_semaphore *sem, int wake_type)
@@ -217,9 +182,6 @@ static void __ldsem_wake_writer(struct ld_semaphore *sem)
return;
} while (1);

- /* reset read lock recursion map */
- ldsem_clear_task_map(sem);
-
/* We must be careful not to touch 'waiter' after we set ->task = NULL.
* It is an allocated on the waiter's stack and may become invalid at
* any time after that point (due to a wakeup from another source).
@@ -268,17 +230,9 @@ down_failed(struct ld_semaphore *sem, unsigned flags, long adjust, long timeout)
/* set up my own style of waitqueue */
raw_spin_lock_irq(&sem->wait_lock);

- if (flags & LDSEM_READ_WAIT) {
- /* Handle recursive read locking -- if the reader already has
- * a read lock then allow lock acquire without waiting
- * but also without waking other waiters
- */
- if (ldsem_lock_recursion(sem)) {
- raw_spin_unlock_irq(&sem->wait_lock);
- return sem;
- }
+ if (flags & LDSEM_READ_WAIT)
list_add_tail(&waiter.list, &sem->read_wait);
- } else
+ else
list_add_tail(&waiter.list, &sem->write_wait);

waiter.task = current;
@@ -358,9 +312,6 @@ static inline int __ldsem_down_read_nested(struct ld_semaphore *sem,
}
}
lock_stat(sem, acquired);
-
- /* used for read lock recursion test */
- ldsem_update_task_map(sem);
return 1;
}

@@ -371,17 +322,9 @@ static inline int __ldsem_down_write_nested(struct ld_semaphore *sem,

lockdep_acquire(sem, subclass, 0, _RET_IP_);

- raw_spin_lock_irq(&sem->wait_lock);
-
count = atomic_long_add_return(LDSEM_WRITE_BIAS,
(atomic_long_t *)&sem->count);
- if (count == LDSEM_WRITE_BIAS) {
- /* reset read lock recursion map */
- ldsem_clear_task_map(sem);
- raw_spin_unlock_irq(&sem->wait_lock);
- } else {
- raw_spin_unlock_irq(&sem->wait_lock);
-
+ if (count != LDSEM_WRITE_BIAS) {
lock_stat(sem, contended);
if (!down_write_failed(sem, timeout)) {
lockdep_release(sem, 1, _RET_IP_);
@@ -414,8 +357,6 @@ int ldsem_down_read_trylock(struct ld_semaphore *sem)
count + LDSEM_READ_BIAS)) {
lockdep_acquire_read(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
-
- ldsem_update_task_map(sem);
return 1;
}
}
@@ -438,21 +379,13 @@ int ldsem_down_write_trylock(struct ld_semaphore *sem)
{
long count;

- raw_spin_lock_irq(&sem->wait_lock);
-
count = atomic_long_cmpxchg(&sem->count, LDSEM_UNLOCKED,
LDSEM_WRITE_BIAS);
if (count == LDSEM_UNLOCKED) {
- /* reset read lock recursion map */
- ldsem_clear_task_map(sem);
-
- raw_spin_unlock_irq(&sem->wait_lock);
-
lockdep_acquire(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
return 1;
}
- raw_spin_unlock_irq(&sem->wait_lock);
return 0;
}

diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index bbefe71..bfbe41a 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -122,8 +122,6 @@ struct ld_semaphore {
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
-#define TASK_MAP_BITS 157
- DECLARE_BITMAP(task_map, TASK_MAP_BITS);
};

extern void __init_ldsem(struct ld_semaphore *sem, const char *name,
--
1.8.1.2


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