Will Deacon <will.deacon@xxxxxxx> writes: > [+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? I don't think you're missing anything :/ Having two flags like you suggest could work. Or you could just make the flag into a nesting counter. Or do you just remove the clearing from spin_lock()? That gets you: spin_lock(&foo); outb(42, port); // io_sync = 1 spin_lock(&bar); ... spin_unlock(&bar); // mb(); io_sync = 0 spin_unlock(&foo); And I/O outside of the lock case: outb(42, port); // io_sync = 1 spin_lock(&bar); ... spin_unlock(&bar); // mb(); io_sync = 0 Extra barriers are not ideal, but the odd spurious mb() might be preferable to doing another compare and branch or increment in every spin_lock()? cheers
