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    Subject[PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do_wake
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    Date
    Doing cmpxchg will cause cache bouncing when checking
    sem->count. This could cause scalability issue
    in a large machine (e.g. a 80 cores box).

    A pre-read of sem->count can mitigate this.

    Signed-off-by: Alex Shi <alex.shi@intel.com>
    Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
    ---
    include/asm-generic/rwsem.h | 8 ++++----
    lib/rwsem.c | 21 +++++++++++++--------
    2 files changed, 17 insertions(+), 12 deletions(-)

    diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
    index bb1e2cd..052d973 100644
    --- a/include/asm-generic/rwsem.h
    +++ b/include/asm-generic/rwsem.h
    @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)

    static inline int __down_write_trylock(struct rw_semaphore *sem)
    {
    - long tmp;
    + if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
    + return 0;

    - tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
    - RWSEM_ACTIVE_WRITE_BIAS);
    - return tmp == RWSEM_UNLOCKED_VALUE;
    + return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
    + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
    }

    /*
    diff --git a/lib/rwsem.c b/lib/rwsem.c
    index 19c5fa9..2072af5 100644
    --- a/lib/rwsem.c
    +++ b/lib/rwsem.c
    @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    * will block as they will notice the queued writer.
    */
    wake_up_process(waiter->task);
    - goto out;
    + return sem;
    }

    /* Writers might steal the lock before we grant it to the next reader.
    @@ -85,15 +85,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    adjustment = 0;
    if (wake_type != RWSEM_WAKE_READ_OWNED) {
    adjustment = RWSEM_ACTIVE_READ_BIAS;
    - try_reader_grant:
    - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
    - if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
    - /* A writer stole the lock. Undo our reader grant. */
    + while (1) {
    + /* A writer stole the lock. */
    + if (sem->count < RWSEM_WAITING_BIAS)
    + return sem;
    +
    + oldcount = rwsem_atomic_update(adjustment, sem)
    + - adjustment;
    + if (likely(oldcount >= RWSEM_WAITING_BIAS))
    + break;
    +
    + /* A writer stole the lock. Undo our reader grant. */
    if (rwsem_atomic_update(-adjustment, sem) &
    RWSEM_ACTIVE_MASK)
    - goto out;
    + return sem;
    /* Last active locker left. Retry waking readers. */
    - goto try_reader_grant;
    }
    }

    @@ -136,7 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    sem->wait_list.next = next;
    next->prev = &sem->wait_list;

    - out:
    return sem;
    }

    --
    1.7.4.4




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