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Subject64-bit ppc rwsem (was: Re: [GIT] Sparc)
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On Tue, 2010-08-17 at 19:14 -0700, David Miller wrote:

> > I merged your pull request, but you've got some fixing up to do,
> > methinks. I also really think you need to make your rwsem's use 64-bit
> > values on sparc64, because otherwise you can overflow the mmap_sem by
> > having more than 65536 threads doing page-faults (on 32-bit, having
> > more than 2**16 threads in one process is unlikely to work for other
> > reasons, like just pure stack usage, so we don't really care about the
> > 32-bit case)
>
> I have a patch to do this already, just need to test it.
>
> You should bug the powerpc folks too :-)

32K threads :-) you guys are nuts !

Here's an untested patch for the folks on linuxppc-dev to look at, I'll
review my own stuff & test tomorrow.

Cheers,
Ben.

powerpc: Make rwsem use "long" types on 64-bit platforms

This should avoid overflow of the mmap_sem when playing with insane
number of threads.

Not-signed-off-by-yet.

diff --git a/arch/powerpc/include/asm/rwsem.h b/arch/powerpc/include/asm/rwsem.h
index 24cd928..ca64a98 100644
--- a/arch/powerpc/include/asm/rwsem.h
+++ b/arch/powerpc/include/asm/rwsem.h
@@ -21,15 +21,20 @@
/*
* the semaphore definition
*/
-struct rw_semaphore {
- /* XXX this should be able to be an atomic_t -- paulus */
- signed int count;
-#define RWSEM_UNLOCKED_VALUE 0x00000000
-#define RWSEM_ACTIVE_BIAS 0x00000001
-#define RWSEM_ACTIVE_MASK 0x0000ffff
-#define RWSEM_WAITING_BIAS (-0x00010000)
+#ifdef CONFIG_PPC64
+# define RWSEM_ACTIVE_MASK 0xffffffffL
+#else
+# define RWSEM_ACTIVE_MASK 0x0000ffffL
+#endif
+
+#define RWSEM_UNLOCKED_VALUE 0x00000000L
+#define RWSEM_ACTIVE_BIAS 0x00000001L
+#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+
+struct rw_semaphore {
+ atomic_long_t count;
spinlock_t wait_lock;
struct list_head wait_list;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -43,9 +48,13 @@ struct rw_semaphore {
# define __RWSEM_DEP_MAP_INIT(lockname)
#endif

-#define __RWSEM_INITIALIZER(name) \
- { RWSEM_UNLOCKED_VALUE, __SPIN_LOCK_UNLOCKED((name).wait_lock), \
- LIST_HEAD_INIT((name).wait_list) __RWSEM_DEP_MAP_INIT(name) }
+#define __RWSEM_INITIALIZER(name) \
+{ \
+ ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE), \
+ __SPIN_LOCK_UNLOCKED((name).wait_lock), \
+ LIST_HEAD_INIT((name).wait_list) \
+ __RWSEM_DEP_MAP_INIT(name) \
+}

#define DECLARE_RWSEM(name) \
struct rw_semaphore name = __RWSEM_INITIALIZER(name)
@@ -70,16 +79,16 @@ extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
*/
static inline void __down_read(struct rw_semaphore *sem)
{
- if (unlikely(atomic_inc_return((atomic_t *)(&sem->count)) <= 0))
+ if (unlikely(atomic_long_inc_return(&sem->count) <= 0))
rwsem_down_read_failed(sem);
}

static inline int __down_read_trylock(struct rw_semaphore *sem)
{
- int tmp;
+ long tmp;

- while ((tmp = sem->count) >= 0) {
- if (tmp == cmpxchg(&sem->count, tmp,
+ while ((tmp = atomic_long_read(&sem->count)) >= 0) {
+ if (tmp == cmpxchg((long *)&sem->count, tmp,
tmp + RWSEM_ACTIVE_READ_BIAS)) {
return 1;
}
@@ -92,10 +101,10 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
*/
static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
{
- int tmp;
+ long tmp;

- tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
- (atomic_t *)(&sem->count));
+ tmp = atomic_long_add_return(RWSEM_ACTIVE_WRITE_BIAS,
+ &sem->count);
if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
rwsem_down_write_failed(sem);
}
@@ -107,9 +116,9 @@ static inline void __down_write(struct rw_semaphore *sem)

static inline int __down_write_trylock(struct rw_semaphore *sem)
{
- int tmp;
+ long tmp;

- tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+ tmp = cmpxchg((long *)&sem->count, RWSEM_UNLOCKED_VALUE,
RWSEM_ACTIVE_WRITE_BIAS);
return tmp == RWSEM_UNLOCKED_VALUE;
}
@@ -119,9 +128,9 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
*/
static inline void __up_read(struct rw_semaphore *sem)
{
- int tmp;
+ long tmp;

- tmp = atomic_dec_return((atomic_t *)(&sem->count));
+ tmp = atomic_long_dec_return(&sem->count);
if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
rwsem_wake(sem);
}
@@ -131,17 +140,17 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- if (unlikely(atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
- (atomic_t *)(&sem->count)) < 0))
+ if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
+ &sem->count) < 0))
rwsem_wake(sem);
}

/*
* implement atomic add functionality
*/
-static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
+static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
{
- atomic_add(delta, (atomic_t *)(&sem->count));
+ atomic_long_add(delta, &sem->count);
}

/*
@@ -149,9 +158,9 @@ static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
*/
static inline void __downgrade_write(struct rw_semaphore *sem)
{
- int tmp;
+ long tmp;

- tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
+ tmp = atomic_long_add_return(-RWSEM_WAITING_BIAS, &sem->count);
if (tmp < 0)
rwsem_downgrade_wake(sem);
}
@@ -159,14 +168,14 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
/*
* implement exchange and add functionality
*/
-static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
+static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
{
- return atomic_add_return(delta, (atomic_t *)(&sem->count));
+ return atomic_long_add_return(delta, &sem->count);
}

static inline int rwsem_is_locked(struct rw_semaphore *sem)
{
- return (sem->count != 0);
+ return atomic_long_read(&sem->count) != 0;
}

#endif /* __KERNEL__ */



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