Re: next-20191122: qemu arm64: WARNING: suspicious RCU usage

[Masami Hiramatsu <mhiramat@kernel.org>]

Hello,

Sorry for replying so late.

On Fri, 22 Nov 2019 22:43:56 +0100
Anders Roxell <anders.roxell@linaro.org> wrote:

> Hi,
> 
> I'm seeing the following warning when I'm booting an arm64 allmodconfig
> kernel [1] on linux-next; tag next-20191122, is this anything you've seen
> before ?
> 
> The code seems to have introduced a long time ago and the warning was
> added recently 28875945ba98 ("rcu: Add support for consolidated-RCU
> reader checking").

I've never seen this but the warning itself is justified.

struct kprobe *get_kprobe(void *addr)
{
        struct hlist_head *head;
        struct kprobe *p;

        head = &kprobe_table[hash_ptr(addr, KPROBE_HASH_BITS)];
        hlist_for_each_entry_rcu(p, head, hlist) {  <---- this cause the warning
                if (p->addr == addr)
                        return p;
        }

        return NULL;
}

The kprobe_table itself is protected by kprobe_mutex OR rcu.
If the caller locks the kprobe_mutex, we can safely access the hash
table, this is what the register_kprobe does. If not, the caller must
disable preemption, this happens when a kprobe (breakpoint) is hit.

Thus, the kernel itself is safe, but it should be fixed.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>