Re: [PATCH] fix a race condition in cancelable mcs spinlocks

Mikulas Patocka
2014-06-04T12:55:00Z

On Tue, 3 Jun 2014, Peter Zijlstra wrote:

> On Mon, Jun 02, 2014 at 12:56:40PM -0700, James Bottomley wrote:
> > Architecturally, there is a way we could emulate the atomic exchange
> > instructions. We could have a special section of memory that always
> > triggers a page trap. In the Q state dtlb trap handlers we could
> > recognise the "atomic" section of memory and wrap the attempted
> > modification in a semaphore. This would add a bit of overhead, but not
> > a huge amount if we do it in the trap handlers like the TMPALIAS
> > flushes. This involves a lot of work for us because we have to decode
> > the instructions in software, recognise the operations and manually
> > apply the hashed semaphores around them. If we did it like this, all
> > we'd need by way of mainline support is that variables treated as
> > atomically exchangeable should be in a separate section (because it's a
> > page fault handler effectively, we need them all separated from "normal"
> > code). This would probably require some type of variable marker and if
> > we ever saw a xchg or cmpxchg on a variable without the marker, we could
> > break the build.
>
> Cute, but I don't think that's entirely feasible given how these things
> can be embedded in other structures (some dynamically allocated etc..).

We could deliberately misalign all the atomic variables - then, we would
take the alignment trap (that is already written) and take the atomic
spinlock in it.

I've got another idea - we could stop the other CPUs while xchg or cmpxchg
is being executed. But there is a problem if the other CPU has interrupts
disabled. Could we mask interrupts on PA-RISC in such a way that they are
all disabled except one IPI that stops the CPU temporarily? Maybe do not
mask interrupts with PSW I-bit and mask them with EIEM instead (leaving
the one interrupt for cmpxchg IPI enabled)?

Mikulas