Re: [RFC PATCH 2/3] futex: Add basic infrastructure for local task local hash.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-10-27 13:19:54 [+0100], Thomas Gleixner wrote:
> > +	if (current->futex_hash_table)
> > +		return -EALREADY;
> 
> You also need to check whether a hash table exists already in the
> process. The table must be process shared to make sense. So it should
> live in current->signal, which is a valid pointer in the context of
> current.
> 
> So this needs some more thoughts especially vs. automatic creation and
> sharing.
> 
> The first question is whether the hash table might have been already
> created when the application reaches main(). If so then this call will
> fail.
> 
> To make this work correctly, this needs proper serialization as well.
> 
> Assume a situation where the application does not allocate a table
> upfront and the first futex use happens late when threads are running
> already.
> 
> CPU0                            CPU1
> T1                              T2        
> sys_futex()                     sys_futex()
>   create_hash()                   create_hash()
> 
> So T1 and T2 create their local hash and the subsequent usage will fail
> because they operate on different hashs. You have the same problem
> vs. your allocation scheme when two threads do prctl(ALLOC). We really
> want to make this as simple as possible.

So I moved this to struct signal_struct and limited allocation to the
group leader.

You want automated creation of this? For everyone or with a hint? This
is 64 bytes per slot due to the cache alignment but event without this
struct takes 56 bytes on PREEMPT_RT and 24 bytes on non-RT. So the four
slots are 256 bytes. Assuming 2.5k tasks it takes 625 KiB. Maybe not
that much.

Let me post v2 the signal_struct and then think about auto ON.

> 
> Thanks,
> 
>         tglx

Sebastian