On 20/01/15 20:12, Waiman Long wrote:
> This patch adds the necessary XEN specific code to allow XEN to
> support the CPU halting and kicking operations needed by the queue
> spinlock PV code.
Xen is a word, please don't capitalize it.
> +void xen_lock_stats(int stat_types)
> +{
> + if (stat_types & PV_LOCKSTAT_WAKE_KICKED)
> + add_smp(&wake_kick_stats, 1);
> + if (stat_types & PV_LOCKSTAT_WAKE_SPURIOUS)
> + add_smp(&wake_spur_stats, 1);
> + if (stat_types & PV_LOCKSTAT_KICK_NOHALT)
> + add_smp(&kick_nohlt_stats, 1);
> + if (stat_types & PV_LOCKSTAT_HALT_QHEAD)
> + add_smp(&halt_qhead_stats, 1);
> + if (stat_types & PV_LOCKSTAT_HALT_QNODE)
> + add_smp(&halt_qnode_stats, 1);
> + if (stat_types & PV_LOCKSTAT_HALT_ABORT)
> + add_smp(&halt_abort_stats, 1);
> +}
> +PV_CALLEE_SAVE_REGS_THUNK(xen_lock_stats);
This is not inlined and the 6 test-and-branch cannot be optimized away.
> +/*
> + * Halt the current CPU & release it back to the host
> + * Return 0 if halted, -1 otherwise.
> + */
> +int xen_halt_cpu(u8 *byte, u8 val)
> +{
> + int irq = __this_cpu_read(lock_kicker_irq);
> + unsigned long flags;
> + u64 start;
> +
> + /* If kicker interrupts not initialized yet, just spin */
> + if (irq == -1)
> + return -1;
> +
> + /*
> + * Make sure an interrupt handler can't upset things in a
> + * partially setup state.
> + */
> + local_irq_save(flags);
> + start = spin_time_start();
> +
> + /* clear pending */
> + xen_clear_irq_pending(irq);
> +
> + /* Allow interrupts while blocked */
> + local_irq_restore(flags);
It's not clear what "partially setup state" is being protected here.
xen_clear_irq_pending() is an atomic bit clear.
I think you can drop the irq save/restore here.
> + /*
> + * Don't halt if the content of the given byte address differs from
> + * the expected value. A read memory barrier is added to make sure that
> + * the latest value of the byte address is fetched.
> + */
> + smp_rmb();
The atomic bit clear in xen_clear_irq_pending() acts as a full memory
barrier. I don't think you need an additional memory barrier here, only
a compiler one. I suggest using READ_ONCE().
> + if (*byte != val) {
> + xen_lock_stats(PV_LOCKSTAT_HALT_ABORT);
> + return -1;
> + }
> + /*
> + * If an interrupt happens here, it will leave the wakeup irq
> + * pending, which will cause xen_poll_irq() to return
> + * immediately.
> + */
> +
> + /* Block until irq becomes pending (or perhaps a spurious wakeup) */
> + xen_poll_irq(irq);
> + spin_time_accum_blocked(start);
> + return 0;
> +}
> +PV_CALLEE_SAVE_REGS_THUNK(xen_halt_cpu);
> +
> +#endif /* CONFIG_QUEUE_SPINLOCK */
> +
> static irqreturn_t dummy_handler(int irq, void *dev_id)
> {
> BUG();
David
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