Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

[Waiman Long <llong@redhat.com>]

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