On Thu, 2009-08-20 at 18:40 +0400, Nikita V. Youshchenko wrote: > > There are three spinning-state structures (per CPU) here, allowing for > > spinning in process context, in bottom-half (softirq) context, and in > > hard-interrupt context. > > There is an interrupt flag (called IRQF_DISABLED in 2.6.26, it was > SA_NODELAY at some point in the past AFAIR) that controls if irq handler > is called immediately with interrupts disabled, or a bit later with this > irq masked and interrupts enabled. > > Most interrupt handlers don't set this flag, so they are called with > interrupts enabled. > > If already spinning, softrq context can't be entered other way that from > after hardirq processing. From this POV, spinlock taken in irq handler and > spinlock taken in softrq handler are equivalent.
Agreed. > And I've just checked, in_interrupt() actually checks for both. > > > Since interrupts are generally enabled during > > interrupt handlers, there need to be multiple levels of hard-interrupt > > contexts (one per IRQ), if I'm not mistaken. The code in SLE 11 and XCI > > appears to implement this. > > I've just looked into code. > > In Xen, hard interrupts are delivered to kernel not by hardware, but by Xen > hypervisor through Xen event channels. At code level, that corresponds to > (async) call to evtchn_do_upcall(). Yes, I understood that. > But that routine has explicit protection against double-entry on the > same cpu, see the code for details. I see. Xen is weird! > That means, do_IRQ can't be called on the same CPU before previous > invocation returns. So multiple levels of "hard-interrupt contexts" just > can't exist. > > Then even 2 spinning-state structures per cpu should be enough. No need to > have more. > > And then my fix looks correct. It still leaves some overkill (3rd > spinning-state), but it should reliably avoid the original crash. OK, agreed. Ben. > Nikita > > P.S. > Btw, server still works without crashes :) > -- Ben Hutchings If at first you don't succeed, you're doing about average.
signature.asc
Description: This is a digitally signed message part
