at 1:13 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> On Wed, Aug 29, 2018 at 07:36:22PM +0000, Nadav Amit wrote:
>> at 10:11 AM, Nadav Amit <namit@xxxxxxxxxx> wrote:
>>
>>> at 1:59 AM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>>
>>>> On Wed, 29 Aug 2018 01:11:42 -0700
>>>> Nadav Amit <namit@xxxxxxxxxx> wrote:
>>>>
>>>>> Use lockdep to ensure that text_mutex is taken when text_poke() is
>>>>> called.
>>>>>
>>>>> Actually it is not always taken, specifically when it is called by kgdb,
>>>>> so take the lock in these cases.
>>>>
>>>> Can we really take a mutex in kgdb context?
>>>>
>>>> kgdb_arch_remove_breakpoint
>>>> <- dbg_deactivate_sw_breakpoints
>>>> <- kgdb_reenter_check
>>>> <- kgdb_handle_exception
>>>> <- __kgdb_notify
>>>> <- kgdb_ll_trap
>>>> <- do_int3
>>>> <- kgdb_notify
>>>> <- die notifier
>>>>
>>>> kgdb_arch_set_breakpoint
>>>> <- dbg_activate_sw_breakpoints
>>>> <- kgdb_reenter_check
>>>> <- kgdb_handle_exception
>>>> ...
>>>>
>>>> Both seems called in exception context, so we can not take a mutex lock.
>>>> I think kgdb needs a special path.
>>>
>>> You are correct, but I don’t want a special path. Presumably text_mutex is
>>> guaranteed not to be taken according to the code.
>>>
>>> So I guess the only concern is lockdep. Do you see any problem if I change
>>> mutex_lock() into mutex_trylock()? It should always succeed, and I can add a
>>> warning and a failure path if it fails for some reason.
>>
>> Err.. This will not work. I think I will drop this patch, since I cannot
>> find a proper yet simple assertion. Creating special path just for the
>> assertion seems wrong.
>
> It's probably worth expanding the comment for text_poke() to call out
> the kgdb case and reference kgdb_arch_{set,remove}_breakpoint(), whose
> code and comments make it explicitly clear why its safe for them to
> call text_poke() without acquiring the lock. Might prevent someone
> from going down this path again in the future.
I thought that the whole point of the patch was to avoid comments, and
instead enforce the right behavior. I don’t understand well enough kgdb
code, so I cannot attest it does the right thing. What happens if
kgdb_do_roundup==0?
