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SubjectRe: [PATCH 2/9] RISC-V: Atomic and Locking Code
On Tue, Jul 04, 2017 at 12:50:55PM -0700, Palmer Dabbelt wrote:
> +/*
> + * FIXME: I could only find documentation that atomic_{add,sub,inc,dec} are
> + * barrier-free. I'm assuming that and/or/xor have the same constraints as the
> + * others.
> + */

Yes.. we have new documentation in the work to which I would post a link
but for some reason copy/paste stopped working again (Konsole does that
at times and is #$%#$%#4# annoying).

Ha, found it using google...

https://marc.info/?l=linux-kernel&m=14972790112580


> +

> +/*
> + * atomic_{cmp,}xchg is required to have exactly the same ordering semantics as
> + * {cmp,}xchg and the operations that return, so they need a barrier. We just
> + * use the other implementations directly.
> + */

cmpxchg triggers an extra rule; all conditional operations only need to
imply barriers on success. So a cmpxchg that fails, need not imply any
ordering what so ever.

> +/*
> + * Our atomic operations set the AQ and RL bits and therefor we don't need to
> + * fence around atomics.
> + */
> +#define __smb_mb__before_atomic() barrier()
> +#define __smb_mb__after_atomic() barrier()

Ah, not quite... you need full barriers here. Because your regular
atomic ops imply no ordering what so ever.

> +/*
> + * These barries are meant to prevent memory operations inside a spinlock from
> + * moving outside of that spinlock. Since we set the AQ and RL bits when
> + * entering or leaving spinlocks, no additional fence needs to be performed.
> + */
> +#define smb_mb__before_spinlock() barrier()
> +#define smb_mb__after_spinlock() barrier()

Also probably not true. I _think_ you want a full barrier here, but
given the total lack of documentation on your end and the fact I've not
yet read the spinlock (which I suppose is below) I cannot yet state
more.


> +
> +/* FIXME: I don't think RISC-V is allowed to perform a speculative load. */
> +#define smp_acquire__after_ctrl_dep() barrier()

That would be a very weird thing to disallow... speculative loads are
teh awesome ;-) Note you can get the very same effect from caches when
your stores are not globally atomic.

> +/*
> + * The RISC-V ISA doesn't support byte or half-word AMOs, so we fall back to a
> + * regular store and a fence here. Otherwise we emit an AMO with an AQ or RL
> + * bit set and allow the microarchitecture to avoid the other half of the AMO.
> + */
> +#define __smp_store_release(p, v) \
> +do { \
> + union { typeof(*p) __val; char __c[1]; } __u = \
> + { .__val = (__force typeof(*p)) (v) }; \
> + compiletime_assert_atomic_type(*p); \
> + switch (sizeof(*p)) { \
> + case 1: \
> + case 2: \
> + smb_mb(); \
> + WRITE_ONCE(*p, __u.__val); \
> + break; \
> + case 4: \
> + __asm__ __volatile__ ( \
> + "amoswap.w.rl zero, %1, %0" \
> + : "+A" (*p), "r" (__u.__val) \
> + : \
> + : "memory"); \
> + break; \
> + case 8: \
> + __asm__ __volatile__ ( \
> + "amoswap.d.rl zero, %1, %0" \
> + : "+A" (*p), "r" (__u.__val) \
> + : \
> + : "memory"); \
> + break; \
> + } \
> +} while (0)
> +
> +#define __smp_load_acquire(p) \
> +do { \
> + union { typeof(*p) __val; char __c[1]; } __u = \
> + { .__val = (__force typeof(*p)) (v) }; \
> + compiletime_assert_atomic_type(*p); \
> + switch (sizeof(*p)) { \
> + case 1: \
> + case 2: \
> + __u.__val = READ_ONCE(*p); \
> + smb_mb(); \
> + break; \
> + case 4: \
> + __asm__ __volatile__ ( \
> + "amoor.w.aq %1, zero, %0" \
> + : "+A" (*p) \
> + : "=r" (__u.__val) \
> + : "memory"); \
> + break; \
> + case 8: \
> + __asm__ __volatile__ ( \
> + "amoor.d.aq %1, zero, %0" \
> + : "+A" (*p) \
> + : "=r" (__u.__val) \
> + : "memory"); \
> + break; \
> + } \
> + __u.__val; \
> +} while (0)

'creative' use of amoswap and amoor :-)

You should really look at a normal load with ordering instruction
though, that amoor.aq is a rmw and will promote the cacheline to
exclusive (and dirty it).

> +/*
> + * Simple spin lock operations. These provide no fairness guarantees.
> + */
> +
> +/* FIXME: Replace this with a ticket lock, like MIPS. */
> +
> +#define arch_spin_lock_flags(lock, flags) arch_spin_lock(lock)
> +#define arch_spin_is_locked(x) ((x)->lock != 0)
> +#define arch_spin_unlock_wait(x) \
> + do { cpu_relax(); } while ((x)->lock)
> +
> +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> +{
> + __asm__ __volatile__ (
> + "amoswap.w.rl x0, x0, %0"
> + : "=A" (lock->lock)
> + :: "memory");
> +}
> +
> +static inline int arch_spin_trylock(arch_spinlock_t *lock)
> +{
> + int tmp = 1, busy;
> +
> + __asm__ __volatile__ (
> + "amoswap.w.aq %0, %2, %1"
> + : "=r" (busy), "+A" (lock->lock)
> + : "r" (tmp)
> + : "memory");
> +
> + return !busy;
> +}
> +
> +static inline void arch_spin_lock(arch_spinlock_t *lock)
> +{
> + while (1) {
> + if (arch_spin_is_locked(lock))
> + continue;
> +
> + if (arch_spin_trylock(lock))
> + break;
> + }
> +}

OK, so back to smp_mb__{before,after}_spinlock(), that wants to order
things like:

wakeup: block:

COND = 1; p->state = UNINTERRUPTIBLE;
smp_mb();
smp_mb__before_spinlock();
spin_lock(&lock); if (!COND)
schedule()
if (p->state & state)
goto out;


And here it is important that the COND store not happen _after_ the
p->state load.

Now, your spin_lock() only implies the AQ thing, which should only
constraint later load/stores but does nothing for the prior load/stores.
So our COND store can drop into the lock and even happen after the
p->state load.

So you very much want your smp_mb__{before,after}_spinlock thingies to
be full barriers.

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