On Wed, 29 Mar 2017 10:18:48 -0700
Josh Stone <jist...@redhat.com> wrote:
> On 03/29/2017 01:25 AM, Masami Hiramatsu wrote:
> > On Wed, 29 Mar 2017 08:30:05 +0200
> > Ingo Molnar <mi...@kernel.org> wrote:
> >>
> >> * Masami Hiramatsu <mhira...@kernel.org> wrote:
> >>
> >>> @@ -1824,6 +1823,30 @@ void unregister_jprobes(struct jprobe **jps, int
> >>> num)
> >>> EXPORT_SYMBOL_GPL(unregister_jprobes);
> >>>
> >>> #ifdef CONFIG_KRETPROBES
> >>> +
> >>> +/* Try to use free instance first, if failed, try to allocate new
> >>> instance */
> >>> +struct kretprobe_instance *kretprobe_alloc_instance(struct kretprobe *rp)
> >>> +{
> >>> + struct kretprobe_instance *ri = NULL;
> >>> + unsigned long flags = 0;
> >>> +
> >>> + raw_spin_lock_irqsave(&rp->lock, flags);
> >>> + if (!hlist_empty(&rp->free_instances)) {
> >>> + ri = hlist_entry(rp->free_instances.first,
> >>> + struct kretprobe_instance, hlist);
> >>> + hlist_del(&ri->hlist);
> >>> + }
> >>> + raw_spin_unlock_irqrestore(&rp->lock, flags);
> >>> +
> >>> + /* Populate max active instance if possible */
> >>> + if (!ri && rp->maxactive < KRETPROBE_MAXACTIVE_ALLOC) {
> >>> + ri = kmalloc(sizeof(*ri) + rp->data_size, GFP_ATOMIC);
> >>> + if (ri)
> >>> + rp->maxactive++;
> >>> + }
> >>> +
> >>> + return ri;
> >>> +}
> >>> /*
> >>> * This kprobe pre_handler is registered with every kretprobe. When probe
> >>> * hits it will set up the return probe.
> >>> @@ -1846,14 +1869,8 @@ static int pre_handler_kretprobe(struct kprobe *p,
> >>> struct pt_regs *regs)
> >>> }
> >>>
> >>> /* TODO: consider to only swap the RA after the last pre_handler fired
> >>> */
> >>> - hash = hash_ptr(current, KPROBE_HASH_BITS);
> >>> - raw_spin_lock_irqsave(&rp->lock, flags);
> >>> - if (!hlist_empty(&rp->free_instances)) {
> >>> - ri = hlist_entry(rp->free_instances.first,
> >>> - struct kretprobe_instance, hlist);
> >>> - hlist_del(&ri->hlist);
> >>> - raw_spin_unlock_irqrestore(&rp->lock, flags);
> >>> -
> >>> + ri = kretprobe_alloc_instance(rp);
> >>> + if (ri) {
> >>> ri->rp = rp;
> >>> ri->task = current;
> >>>
> >>> @@ -1868,13 +1885,13 @@ static int pre_handler_kretprobe(struct kprobe
> >>> *p, struct pt_regs *regs)
> >>>
> >>> /* XXX(hch): why is there no hlist_move_head? */
> >>> INIT_HLIST_NODE(&ri->hlist);
> >>> + hash = hash_ptr(current, KPROBE_HASH_BITS);
> >>> kretprobe_table_lock(hash, &flags);
> >>> hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
> >>> kretprobe_table_unlock(hash, &flags);
> >>> - } else {
> >>> + } else
> >>> rp->nmissed++;
> >>> - raw_spin_unlock_irqrestore(&rp->lock, flags);
> >>> - }
> >>> +
> >>> return 0;
> >>> }
> >>> NOKPROBE_SYMBOL(pre_handler_kretprobe);
> >>
> >> So this is something I missed while the original code was merged, but the
> >> concept
> >> looks a bit weird: why do we do any "allocation" while a handler is
> >> executing?
> >>
> >> That's fundamentally fragile. What's the maximum number of parallel
> >> 'kretprobe_instance' required per kretprobe - one per CPU?
> >
> > It depends on the place where we put the probe. If the probed function will
> > be
> > blocked (yield to other tasks), then we need a same number of threads on
> > the system which can invoke the function. So, ultimately, it is same
> > as function_graph tracer, we need it for each thread.
>
> Isn't it also possible that the function may be reentrant? Whether by
> plain recursion or an interrupt call, this leads to multiple live
> instances even for a given thread.
Yes, that's another possible case, but I don't think that's so serious in kernel
because we have very limited kernel stack, which means the recursion may not
so deep.
Thank you,
--
Masami Hiramatsu <mhira...@kernel.org>
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