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    SubjectRe: [PATCH 4/4] locking/rwbase: Take care of ordering guarantee for fastpath reader
    Date
    On Thu, Sep 09 2021 at 12:59, Peter Zijlstra wrote:
    > 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;
    > }

    Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

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