On 10/1/20 10:27 AM, Oleg Nesterov wrote:
> Jens,
>
> I'll read this version tomorrow, but:
>
> On 10/01, Jens Axboe wrote:
>>
>> static inline int signal_pending(struct task_struct *p)
>> {
>> - return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
>> +#ifdef TIF_TASKWORK
>> + /*
>> + * TIF_TASKWORK isn't really a signal, but it requires the same
>> + * behavior of restarting the system call to force a kernel/user
>> + * transition.
>> + */
>> + return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING) ||
>> + test_tsk_thread_flag(p, TIF_TASKWORK));
>> +#else
>> + return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING));
>> +#endif
>
> This change alone is already very wrong.
>
> signal_pending(task) == T means that this task will do get_signal() as
> soon as it can, and this basically means you can't "divorce" SIGPENDING
> and TASKWORK.
>
> Simple example. Suppose we have a single-threaded task T.
>
> Someone does task_work_add(T, TWA_SIGNAL). This makes signal_pending()==T
> and this is what we need.
>
> Now suppose that another task sends a signal to T before T calls
> task_work_run() and clears TIF_TASKWORK. In this case SIGPENDING won't
> be set because signal_pending() is already set (see wants_signal), and
> this means that T won't notice this signal.
That's a good point, and I have been thinking along those lines. The
"problem" is the two different use cases:
1) The "should I return from schedule() or break out of schedule() loops
kind of use cases".
2) Internal signal delivery use cases.
The former wants one that factors in TIF_TASKWORK, while the latter
should of course only look at TIF_SIGPENDING.
Now, my gut reaction would be to have __signal_pending() that purely
checks for TIF_SIGPENDING, and make sure we use that on the signal
delivery side of things. Or something with a better name than that, but
functionally the same. Ala:
static inline int __signal_pending(struct task_struct *p)
{
return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING));
}
static inline int signal_pending(struct task_struct *p)
{
#ifdef TIF_TASKWORK
return unlikely(test_tsk_thread_flag(p, TIF_TASKWORK)||
__signal_pending(p));
#else
return __signal_pending(p));
#endif
}
and then use __signal_pending() on the signal delivery side.
It's still not great in the sense that renaming signal_pending() would
be a better choice, but that's a whole lot of churn...
--
Jens Axboe
