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

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Message-ID: <20250107135106.WWrtBMXY@linutronix.de>
Date: Tue, 7 Jan 2025 14:51:06 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Wander Lairson Costa <wander@...hat.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jeff Garzik <jgarzik@...hat.com>,
	Auke Kok <auke-jan.h.kok@...el.com>,
	"moderated list:INTEL ETHERNET DRIVERS" <intel-wired-lan@...ts.osuosl.org>,
	"open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:Real-time Linux (PREEMPT_RT):Keyword:PREEMPT_RT" <linux-rt-devel@...ts.linux.dev>
Subject: Re: [PATCH iwl-net 0/4] igb: fix igb_msix_other() handling for
 PREEMPT_RT

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