Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers

[NeilBrown]

On Wed, Dec 09 2015, Peter Zijlstra wrote:

> On Wed, Dec 09, 2015 at 12:06:33PM +1100, NeilBrown wrote:
>> On Tue, Dec 08 2015, Peter Zijlstra wrote:
>> 
>> >>  
>> >
>> > *sigh*, so that patch was broken.. the below might fix it, but please
>> > someone look at it, I seem to have a less than stellar track record
>> > here...
>> 
>> This new change seems to be more intrusive than should be needed.
>> Can't we just do:
>> 
>> 
>>  __sched int bit_wait(struct wait_bit_key *word)
>>  {
>> +	long state = current->state;
>
> No, current->state can already be changed by this time.

Does that matter?
It can only have changed to TASK_RUNNING - right?
In that case signal_pending_state() will return 0 and the bit_wait() acts
as though the thread was woken up normally (which it was) rather than by
a signal (which maybe it was too, but maybe that happened just a tiny
bit later).

As long as signal delivery doesn't change ->state, we should be safe.
We should even be safe testing ->state *after* the call the schedule().

NeilBrown

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