Messages in this thread | | | From | Waiman Long <> | Subject | Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader | Date | Wed, 1 Sep 2021 14:53:37 -0400 |
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On 9/1/21 11:06 AM, Boqun Feng wrote: > 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> > --- > Hi Thomas and Peter, > > Sorry I'm late for the party of PREEMPT_RT lock review. Just want to > point the problem with this patch. Not even compile test, but show the > idea and check if I'm missing something subtle. > > Regards, > Boqun > > > kernel/locking/rwbase_rt.c | 34 ++++++++++++++++++++++++++++++---- > 1 file changed, 30 insertions(+), 4 deletions(-) > > diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c > index 4ba15088e640..a1886fd8bde6 100644 > --- 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 then should be stronger than _acquire() > + * and _release() to provide necessary ordering guarantee. > + * > * Common code shared between RT rw_semaphore and rwlock > */ > > @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) > * 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)))
Should we also change *cmpxchg() to cmpxchg_acquire() as it is a little cheaper for ll/sc arches?
The other changes look good to me.
Cheers, Longman
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