Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 23, 2016, at 5:26 PM, Dave Watson davejwatson@fb.com wrote:

> Hi Mathieu,

> > Implements two basic tests of RSEQ functionality, and one more
> > exhaustive parameterizable test.

> Thanks for beefing up the tests. I ran this set through our jemalloc
> tests using rseq, and everything looks good so far.

> +static inline __attribute__((always_inline))
> +bool rseq_finish(struct rseq_lock *rlock,
> + intptr_t *p, intptr_t to_write,
> + struct rseq_state start_value)
> +{
> + RSEQ_INJECT_C(9)
> +
> + if (unlikely(start_value.lock_state != RSEQ_LOCK_STATE_RESTART)) {
> + if (start_value.lock_state == RSEQ_LOCK_STATE_LOCK)
> + rseq_fallback_wait(rlock);
> + return false;
> + }
> +
> +#ifdef __x86_64__
> + /*
> + * The __rseq_table section can be used by debuggers to better
> + * handle single-stepping through the restartable critical
> + * sections.
> + */
> + __asm__ __volatile__ goto (
> + ".pushsection __rseq_table, \"aw\"\n\t"
> + ".balign 8\n\t"
> + "4:\n\t"
> + ".quad 1f, 2f, 3f\n\t"
> + ".popsection\n\t"

> Is there a reason we're also passing the start ip? It looks unused.
> I see the "for debuggers" comment, but it looks like all the debugger
> support is done in userspace.

> + "1:\n\t"
> + RSEQ_INJECT_ASM(1)
> + "movq $4b, (%[rseq_cs])\n\t"
> + RSEQ_INJECT_ASM(2)
> + "cmpl %[start_event_counter], %[current_event_counter]\n\t"
> + "jnz 3f\n\t"
> + RSEQ_INJECT_ASM(3)
> + "movq %[to_write], (%[target])\n\t"
> + "2:\n\t"
> + RSEQ_INJECT_ASM(4)
> + "movq $0, (%[rseq_cs])\n\t"
> + "jmp %l[succeed]\n\t"
> + "3: movq $0, (%[rseq_cs])\n\t"
> + : /* no outputs */
> + : [start_event_counter]"r"(start_value.event_counter),
> + [current_event_counter]"m"(start_value.rseqp->abi.u.e.event_counter),
> + [to_write]"r"(to_write),
> + [target]"r"(p),
> + [rseq_cs]"r"(&start_value.rseqp->abi.rseq_cs)
> + RSEQ_INJECT_INPUT
> + : "memory", "cc"
> + RSEQ_INJECT_CLOBBER
> + : succeed
> + );

> This ABI looks like it will work fine for our use case. I don't think it
> has been mentioned yet, but we may still need multiple asm blocks
> for differing numbers of writes. For example, an array-based freelist push:

> void push(void *obj) {
> if (index < maxlen) {
> freelist[index++] = obj;
> }
> }

> would be more efficiently implemented with a two-write rseq_finish:

> rseq_finish2(&freelist[index], obj, // first write
> &index, index + 1, // second write
> ...);

> where it is ok to abort between the two writes, but both need to happen
> on the same cpu.

(re-send without html formatting for the mailing lists)

Would pairing one rseq_start with two rseq_finish do the trick
there ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com