Will Deacon <will.dea...@arm.com> 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
