Re: [PATCH v4 0/3] Optimize CAS [BZ #28537]

[Florian Weimer <fweimer@redhat.com>]

* Paul A. Clarke:

> On Tue, Nov 09, 2021 at 04:16:11PM -0800, H.J. Lu via Libc-alpha wrote:
>> CAS instruction is expensive.  From the x86 CPU's point of view, getting
>> a cache line for writing is more expensive than reading.  See Appendix
>> A.2 Spinlock in:
>> 
>> https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf
>> 
>> The full compare and swap will grab the cache line exclusive and cause
>> excessive cache line bouncing.
>> 
>> Optimize CAS in low level locks and pthread_mutex_lock.c:
>> 
>> 1. Do an atomic load and skip CAS if compare may fail to reduce cache
>> line bouncing on contended locks.
>> 2. Replace atomic_compare_and_exchange_bool_acq with
>> atomic_compare_and_exchange_val_acq to avoid the extra load.
>> 3. Drop __glibc_unlikely in __lll_trylock and lll_cond_trylock since we
>> don't know if it's actually rare; in the contended case it is clearly not
>> rare.
>
> I see build errors:
>
> In file included from pthread_mutex_cond_lock.c:23:
> ../nptl/pthread_mutex_lock.c: In function ‘__pthread_mutex_cond_lock_full’:
> ../nptl/pthread_mutex_lock.c:442:6: error: a label can only be part of a statement and a declaration is not a statement
>       int private = (robust
>       ^~~

The patch has:

+ locked_mutex:
 	    /* The mutex is locked.  The kernel will now take care of
 	       everything.  */
 	    int private = (robust

This is only supported in recent C versions, I think the workaround is
to add an empty statement with a semicolon, like this:

+ locked_mutex:;
 	    /* The mutex is locked.  The kernel will now take care of
 	       everything.  */
 	    int private = (robust

Thanks,
Florian