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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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