On Sat, Jul 26 2025 at 09:59, Greg Kroah-Hartman wrote:
On Sat, Jul 26, 2025 at 04:44:42PM +0900, Tetsuo Handa wrote:
static void __usb_hcd_giveback_urb(struct urb *urb) { (...snipped...) kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum) { if (in_serving_softirq()) { local_irq_save(flags); // calling local_irq_save() is wrong if CONFIG_PREEMPT_RT=y kcov_remote_start_usb(id) { kcov_remote_start(id) { kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id)) { (...snipped...) local_lock_irqsave(&kcov_percpu_data.lock, flags) { __local_lock_irqsave(lock, flags) { #ifndef CONFIG_PREEMPT_RT https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_i... #else https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_i... // not calling local_irq_save(flags) #endif
Right, it does not invoke local_irq_save(flags), but it takes the underlying lock, which means it prevents reentrance.
Ok, but then how does the big comment section for kcov_remote_start_usb_softirq() work, where it explicitly states:
- Disables interrupts for the duration of the coverage collection section.
- This allows avoiding nested remote coverage collection sections in the
- softirq context (a softirq might occur during the execution of a work in
- the BH workqueue, which runs with in_serving_softirq() > 0).
- For example, usb_giveback_urb_bh() runs in the BH workqueue with
- interrupts enabled, so __usb_hcd_giveback_urb() might be interrupted in
- the middle of its remote coverage collection section, and the interrupt
- handler might invoke __usb_hcd_giveback_urb() again.
You are removing half of this function entirely, which feels very wrong to me as any sort of solution, as you have just said that all of that documentation entry is now not needed.
I'm not so sure because kcov_percpu_data.lock is only held within kcov_remote_start() and kcov_remote_stop(), but the above comment suggests that the whole section needs to be serialized.
Though I'm not a KCOV wizard and might be completely wrong here.
If the whole section is required to be serialized, then this need another local lock in kcov_percpu_data to work correctly on RT.
Thanks,
tglx