Oleg Nesterov <oleg@xxxxxxxxxx> writes:
> Hi Mike,
>
> sorry, but somehow I can't understand this patch...
>
> I'll try to read it with a fresh head on Weekend, but for example,
>
> On 06/01, Mike Christie wrote:
>>
>> static int vhost_task_fn(void *data)
>> {
>> struct vhost_task *vtsk = data;
>> - int ret;
>> + bool dead = false;
>> +
>> + for (;;) {
>> + bool did_work;
>> +
>> + /* mb paired w/ vhost_task_stop */
>> + if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
>> + break;
>> +
>> + if (!dead && signal_pending(current)) {
>> + struct ksignal ksig;
>> + /*
>> + * Calling get_signal will block in SIGSTOP,
>> + * or clear fatal_signal_pending, but remember
>> + * what was set.
>> + *
>> + * This thread won't actually exit until all
>> + * of the file descriptors are closed, and
>> + * the release function is called.
>> + */
>> + dead = get_signal(&ksig);
>> + if (dead)
>> + clear_thread_flag(TIF_SIGPENDING);
>
> this can't be right or I am totally confused.
>
> Another signal_wake_up() can come right after clear(SIGPENDING).
Technically yes.
However please not that prepare_signal does:
if (signal->flags & SIGNAL_GROUP_EXIT)
return false;
In general it is wrong to receive or attempt to process a signal
after task death has been decided.
Strictly speaking that doesn't cover de_thread, and coredumping
but still receiving any kind of signal at that point is rare
and generally wrong behavior.
Beyond that clearing TIF_SIGPENDING is just an optimization so
the thread can sleep in schedule and not spin.
> Again, I'll try to re-read this patch, but let me ask anyway...
>
> Do we have a plan B? I mean... iirc you have mentioned that you can
> change these code paths to do something like
>
> if (killed)
> tell_the_drivers_that_all_callbacks_will_fail();
>
>
> so that vhost_worker() can exit after get_signal() returns SIGKILL.
>
> Probably I misunderstood you, but it would be nice to avoid the changes
> in coredump/etc code just to add a temporary (iiuc!) fix.
One saving grace with the the vhost code is that you need to open
device nodes that normally have root-only permissions.
If we are willing to allow races in process shutdown to cause leaks I
think we can do something better, and put the burden of work on vhost
layer.
I will follow up with a patch doing that.
Eric
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