Messages in this thread | | | Date | Thu, 21 Jun 2007 23:06:30 +0200 | From | Ingo Molnar <> | Subject | Re: [BUG] long freezes on thinkpad t60 |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Thu, 21 Jun 2007, Ingo Molnar wrote: > > > > 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 :-/ > > I'm fine with a delay, but the __delay(1) is simply not "correct". It > doesn't do anything.
it's a bit trickier than that. Yes, it's a simple 1-entry loop and thus makes little sense to call. But it's a loop that got boot-time calibrated, so we can do this in the spinlock-debug code:
u64 loops = loops_per_jiffy * HZ;
this guarantees that we will loop for _at least_ 1 second before printing a message. (in practice it's much longer, especially with the current naive trylock approach)
Why? Because as part of the activities that the spin-loop does, we also do everything that an mdelay(1000) would do. We do it 'piecemail-wise', and we do it very inefficiently, but the lower time-bound should be guaranteed. This is done because most of the problems were caused by too short looping and bogus debug printouts. So this is basically an open-coded udelay implementation.
Furthermore, with the spin_is_locked() fix i just sent, the __loop(1) solution should actually be quite close to a real udelay() thing, without the delay effect. It would probably more accurate to increase it to loops_per_jiffy*10*HZ/16 and to call a __loop(16) thing, to get closer timings. (but then some people would argue 'why dont you take the lock as soon as it's released'.)
Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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