RE: Assuming that TAS() will succeed the first time is verboten

Ok.

> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: Thursday, December 28, 2000 2:07 PM
> To: Mikheev, Vadim
> Cc: pgsql-hackers@postgreSQL.org
> Subject: Re: Assuming that TAS() will succeed the first time 
> is verboten
> 
> 
> 
> "Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
> > Anyway I don't object if it bothers you -:)
> > But please do not use S_LOCK - as you see WAL code try to do other
> > things if slock of "primary interest" is busy.
> 
> In some places, yes.  But I also saw a number of places where 
> S_LOCK is
> sufficient, and I think it's clearer to code that way whenever there's
> not useful work to do before acquiring the lock.  For example, is
> 
>     if (updrqst)
>     {
>         unsigned    i = 0;
> 
>         for (;;)
>         {
>             if (!TAS(&(XLogCtl->info_lck)))
>             {
>                 if (XLByteLT(XLogCtl->LgwrRqst.Write, LgwrRqst.Write))
>                     XLogCtl->LgwrRqst.Write = LgwrRqst.Write;
>                 S_UNLOCK(&(XLogCtl->info_lck));
>                 break;
>             }
>             s_lock_sleep(i++);
>         }
>     }
> 
> really better than
> 
>     if (updrqst)
>     {
>         S_LOCK(&(XLogCtl->info_lck));
>         if (XLByteLT(XLogCtl->LgwrRqst.Write, LgwrRqst.Write))
>             XLogCtl->LgwrRqst.Write = LgwrRqst.Write;
>         S_UNLOCK(&(XLogCtl->info_lck));
>     }
> 
> ?  I don't think so...
> 
> What I'm thinking of doing for the places where there is useful work
> to do while waiting is
> 
>     spins = 0;
>     while (TAS(lock))
>     {
>         /* do useful work here */
>         S_LOCK_SLEEP(spins++);
>     }
> 
> where S_LOCK_SLEEP() expands to do the same things as the body of the
> existing loop in s_lock().
> 
>             regards, tom lane
>