linux-kernel - Re: [BUG] usbip: vhci: Sleeping function called from invalid context in vhci_urb_enqueue on PREEMPT

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Message-ID: <20250819110457.I46wiKTe@linutronix.de>
Date: Tue, 19 Aug 2025 13:04:57 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Yunseong Kim <ysk@...lloc.com>, linux-usb@...r.kernel.org,
	gregkh@...uxfoundation.org,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Shuah Khan <skhan@...uxfoundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller@...glegroups.com
Subject: Re: [BUG] usbip: vhci: Sleeping function called from invalid context
 in vhci_urb_enqueue on PREEMPT_RT

On 2025-08-17 10:27:11 [-0400], Alan Stern wrote:
> On Sat, Aug 16, 2025 at 10:16:34AM -0400, Alan Stern wrote:
> > So it looks like we should be using a different function instead of 
> > local_irq_disable().  We need something which in a non-RT build will 
> > disable interrupts on the local CPU, but in an RT build will merely 
> > disable preemption.  (In fact, every occurrence of local_irq_disable() 
> > in the USB subsystem probably should be changed in this way.)
> 
> Or maybe what we need is something that in a non-RT build will disable 
> local interrupts and in an RT build will do nothing.  (I suspect that RT 
> kernels won't like it if we call spin_lock() while preemption is 
> disabled.)

This is the local_irq_disable() in vhci_urb_enqueue() before
usb_hcd_giveback_urb() is invoked. It was added in 9e8586827a706
("usbip: vhci_hcd: fix calling usb_hcd_giveback_urb() with irqs
enabled").
The warning that fixed back then was 
|         if (WARN_ON(in_task() && kcov_mode_enabled(mode))) {
which was kernel/kcov.c:834 as of v5.9-rc8 (as of report the mentioned
in the commit).
local_irq_disable() does not change the preemption counter so I am a bit
puzzled why this did shut the warning.

> > Is there such a function?

We could use some API that accidentally does what you ask for. There
would be local_lock_t where local_lock_irq() does that.
What about moving the completion callback to softirq by setting HCD_BH?

> Alan Stern

Sebastian