Messages in this thread | | | Date | Mon, 2 Dec 2019 18:35:31 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe |
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On Tue, Dec 03, 2019 at 07:34:53AM +0900, Masami Hiramatsu wrote: > Hi Joel, > > On Mon, 2 Dec 2019 16:08:54 -0500 > Joel Fernandes <joel@joelfernandes.org> wrote: > > > On Mon, Dec 02, 2019 at 04:32:13PM +0900, Masami Hiramatsu wrote: > > > Anders reported that the lockdep warns that suspicious > > > RCU list usage in register_kprobe() (detected by > > > CONFIG_PROVE_RCU_LIST.) This is because get_kprobe() > > > access kprobe_table[] by hlist_for_each_entry_rcu() > > > without rcu_read_lock. > > > > > > If we call get_kprobe() from the breakpoint handler context, > > > it is run with preempt disabled, so this is not a problem. > > > But in other cases, instead of rcu_read_lock(), we locks > > > kprobe_mutex so that the kprobe_table[] is not updated. > > > So, current code is safe, but still not good from the view > > > point of RCU. > > > > > > Let's lock the rcu_read_lock() around get_kprobe() and > > > ensure kprobe_mutex is locked at those points. > > > > > > Note that we can safely unlock rcu_read_lock() soon after > > > accessing the list, because we are sure the found kprobe has > > > never gone before unlocking kprobe_mutex. Unless locking > > > kprobe_mutex, caller must hold rcu_read_lock() until it > > > finished operations on that kprobe. > > > > > > Reported-by: Anders Roxell <anders.roxell@linaro.org> > > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org> > > > > Instead of this, can you not just pass the lockdep_is_held() expression as > > the last argument to list_for_each_entry_rcu() to silence the warning? Then > > it will be a simpler patch. > > Ah, I see. That is more natural to silence the warning.
Np, and on such fix, my:
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
thanks,
- Joel
> > Thank you! > > -- > Masami Hiramatsu <mhiramat@kernel.org>
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