Re: [PATCH iwl-net 0/4] igb: fix igb_msix_other() handling for PREEMPT_RT

[Sebastian Andrzej Siewior]

On 2024-12-04 08:42:23 [-0300], Wander Lairson Costa wrote:
> This is the second attempt at fixing the behavior of igb_msix_other()
> for PREEMPT_RT. The previous attempt [1] was reverted [2] following
> concerns raised by Sebastian [3].
> 
> The initial approach proposed converting vfs_lock to a raw_spinlock,
> a minor change intended to make it safe. However, it became evident
> that igb_rcv_msg_from_vf() invokes kcalloc with GFP_ATOMIC,
> which is unsafe in interrupt context on PREEMPT_RT systems.
> 
> To address this, the solution involves splitting igb_msg_task()
> into two parts:
> 
>     * One part invoked from the IRQ context.
>     * Another part called from the threaded interrupt handler.
> 
> To accommodate this, vfs_lock has been restructured into a double
> lock: a spinlock_t and a raw_spinlock_t. In the revised design:
> 
>     * igb_disable_sriov() locks both spinlocks.
>     * Each part of igb_msg_task() locks the appropriate spinlock for
>     its execution context.

- Is this limited to PREEMPT_RT or does it also occur on PREEMPT systems
  with threadirqs? And if this is PREEMPT_RT only, why?

- What causes the failure? I see you reworked into two parts to behave
  similar to what happens without threaded interrupts. There is still no
  explanation for it. Is there a timing limit or was there another
  register operation which removed the mailbox message?

> Cheers,
> Wander

Sebastian