* Linus Torvalds <[EMAIL PROTECTED]> wrote: > > for (;;) { > > for (i = 0; i < loops; i++) { > > if (__raw_write_trylock(&lock->raw_lock)) > > return; > > __delay(1); > > } > > What a piece of crap. > > Anybody who ever waits for a lock by busy-looping over it is BUGGY, > dammit! > > The only correct way to wait for a lock is: > > (a) try it *once* with an atomic r-m-w > (b) loop over just _reading_ it (and something that implies a memory > barrier, _not_ "__delay()". Use "cpu_relax()" or "smp_rmb()") > (c) rinse and repeat.
damn, i first wrote up an explanation about why that ugly __delay(1) is there (it almost hurts my eyes when i look at it!) but then deleted it as superfluous :-/ really, it's not because i'm stupid (although i might still be stupid for other resons ;-), it wasnt there in earlier spin-debug versions. We even had an inner spin_is_locked() loop at a stage (and should add it again). the reason for the __delay(1) was really mundane: to be able to figure out when to print a 'we locked up' message to the user. If it's 1 second, it causes false positive on some systems. If it's 10 minutes, people press reset before we print out any useful data. It used to be just a loop of rep_nop()s, but that was hard to calibrate: on certain newer hardware it was triggering as fast as in 2 seconds, causing many false positives. We cannot use jiffies nor any other clocksource in this debug code. so i settled for the butt-ugly but working __delay(1) thing, to be able to time the debug messages. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/