Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH 4/4] locking/rwbase: Take care of ordering guarantee for fastpath reader | Date | Tue, 14 Sep 2021 09:46:23 +0200 |
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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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