[+more ppc folks] On Mon, Feb 18, 2019 at 04:50:12PM +0000, Will Deacon wrote: > On Wed, Feb 13, 2019 at 10:27:09AM -0800, Linus Torvalds wrote: > > Note that even if mmiowb() is expensive (and I don't think that's > > actually even the case on ia64), you can - and probably should - do > > what PowerPC does. > > > > Doing an IO barrier on PowerPC is insanely expensive, but they solve > > that simply track the whole "have I done any IO" manually. It's not > > even that expensive, it just uses a percpu flag. > > > > (Admittedly, PowerPC makes it less obvious that it's a percpu variable > > because it's actually in the special "paca" region that is like a > > hyper-local percpu area). [...] > > But we *could* first just do the mmiowb() unconditionally in the ia64 > > unlocking code, and then see if anybody notices? > > I'll hack this up as a starting point. We can always try to be clever later > on if it's deemed necessary. Ok, so I started hacking this up in core code with the percpu flag (since riscv apparently needs it), but I've now realised that I don't understand how the PowerPC trick works after all. Consider the following: spin_lock(&foo); // io_sync = 0 outb(42, port); // io_sync = 1 spin_lock(&bar); // io_sync = 0 ... spin_unlock(&bar); spin_unlock(&foo); The inner lock could even happen in an irq afaict, but we'll end up skipping the mmiowb()/sync because the io_sync flag is unconditionally cleared by spin_lock(). Fixing this is complicated by the fact that I/O writes can be performed in preemptible context with no locks held, so we can end up spuriously setting the io_sync flag for arbitrary CPUs, hence the desire to clear it in spin_lock(). If the paca entry was more than a byte, we could probably track that a spinlock is held and then avoid clearing the flag prematurely, but I have a feeling that I'm missing something. Anybody know how this is supposed to work? Will
