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    Subject[PATCH 4/4] locking/rwbase: Take care of ordering guarantee for fastpath reader
    From: Boqun Feng <boqun.feng@gmail.com>

    Readers of rwbase can lock and unlock without taking any inner lock, if
    that happens, we need the ordering provided by atomic operations to
    satisfy the ordering semantics of lock/unlock. Without that, considering
    the follow case:

    { X = 0 initially }

    CPU 0 CPU 1
    ===== =====
    rt_write_lock();
    X = 1
    rt_write_unlock():
    atomic_add(READER_BIAS - WRITER_BIAS, ->readers);
    // ->readers is READER_BIAS.
    rt_read_lock():
    if ((r = atomic_read(->readers)) < 0) // True
    atomic_try_cmpxchg(->readers, r, r + 1); // succeed.
    <acquire the read lock via fast path>

    r1 = X; // r1 may be 0, because nothing prevent the reordering
    // of "X=1" and atomic_add() on CPU 1.

    Therefore audit every usage of atomic operations that may happen in a
    fast path, and add necessary barriers.

    Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    kernel/locking/rwbase_rt.c | 21 +++++++++++++++++++--
    1 file changed, 19 insertions(+), 2 deletions(-)

    --- a/kernel/locking/rwbase_rt.c
    +++ b/kernel/locking/rwbase_rt.c
    @@ -41,6 +41,12 @@
    * The risk of writer starvation is there, but the pathological use cases
    * which trigger it are not necessarily the typical RT workloads.
    *
    + * Fast-path orderings:
    + * The lock/unlock of readers can run in fast paths: lock and unlock are only
    + * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
    + * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
    + * and _release() (or stronger).
    + *
    * Common code shared between RT rw_semaphore and rwlock
    */

    @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_t
    * set.
    */
    for (r = atomic_read(&rwb->readers); r < 0;) {
    + /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
    if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
    return 1;
    }
    @@ -162,6 +169,8 @@ static __always_inline void rwbase_read_
    /*
    * rwb->readers can only hit 0 when a writer is waiting for the
    * active readers to leave the critical section.
    + *
    + * dec_and_test() is fully ordered, provides RELEASE.
    */
    if (unlikely(atomic_dec_and_test(&rwb->readers)))
    __rwbase_read_unlock(rwb, state);
    @@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock
    {
    struct rt_mutex_base *rtm = &rwb->rtmutex;

    - atomic_add(READER_BIAS - bias, &rwb->readers);
    + /*
    + * _release() is needed in case that reader is in fast path, pairing
    + * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
    + */
    + (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
    raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
    rwbase_rtmutex_unlock(rtm);
    }
    @@ -201,7 +214,11 @@ static inline bool __rwbase_write_tryloc
    /* Can do without CAS because we're serialized by wait_lock. */
    lockdep_assert_held(&rwb->rtmutex.wait_lock);

    - if (!atomic_read(&rwb->readers)) {
    + /*
    + * _acquire is needed in case the reader is in the fast path, pairing
    + * with rwbase_read_unlock(), provides ACQUIRE.
    + */
    + if (!atomic_read_acquire(&rwb->readers)) {
    atomic_set(&rwb->readers, WRITER_BIAS);
    return 1;
    }

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