From: Matthew Wilcox > Sent: 19 September 2023 16:47 > > On Tue, Sep 19, 2023 at 03:22:25PM +0000, David Laight wrote: > > > Anyway, that's not the brief. We're looking to (eg) clear bit 0 > > > and test whether bit 7 was set. So it's the sign bit of the byte, > > > not the sign bit of the int. > > > > Use the address of the byte as an int and xor with 1u<<24. > > The xor will do a rmw on the three bytes following, but I > > doubt that matters. > > Bet you a shiny penny that Coldfire takes an unaligned access trap ... and then the 'firmware' silently fixed it up for you a few 1000 clocks later... > and besides, this is done on _every_ call to unlock_page(). That might > cross not only a cacheline boundary but also a page boundary. I cannot > believe that would be a high-performing solution. It might be just fine > on m68000 but I bet even by the 030 it's lower performing. I do remember managing to use 'cas2' to add an item to a linked list. But it is so painful so setup it was better just to disable interrupts. For non-smp that is almost certainly ok. (Unless the instructions are slow because of synchronisation.) Otherwise you need to use 'cas' on the aligned word. Assuming coldfire even has cas. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
