On Thu, 07 Dec 2006 20:06:39 +0000 David Howells <[EMAIL PROTECTED]> wrote:
> Andrew Morton <[EMAIL PROTECTED]> wrote: > > > and we can assume (and ensure) that a failing test_and_set_bit() will not > > write to the affected word at all. > > You may not assume that; and indeed that is not so in the generic > spinlock-based bitops or ARM pre-v6 or PA-RISC or sparc32 or ... Ah. How obnoxious of them. > Remember: if you have to put a conditional jump in there, it's going to fail > one way or the other a certain percentage of the time, and that's going to > cause a pipeline stall, and these ops are used quite a lot. > > OTOH, I don't know that the stall would be that bad since the spin_lock and > spin_unlock may cause a stall anyway. > Yes, the branch would cost. But in not uncommon cases that branch will save the machine from dirtying a cacheline. And if we add those branches, we bring those architectures' semantics in line with all the other architectures. And we get better semantics overall. So I don't think we should rule this out. - To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
