On Thu, 27 Nov 2014, Daniel Thompson wrote:
It is currently possible for FIQ handlers to re-enter gic_raise_softirq() and lock up.
gic_raise_softirq() lock(x);-~-> FIQ handle_fiq() gic_raise_softirq() lock(x); <-- Lockup
Calling printk() from a FIQ handler can trigger this problem because printk() raises an IPI when it needs to wake_up_klogd(). More generally, IPIs are the only means for FIQ handlers to safely defer work to less restrictive calling context so the function to raise them really needs to be FIQ-safe.
That's not really true. irq_work can be used from FIQ/NMI context and it was specifically designed for that purpose.
Now printk is a different issue, but there is work in progress to make printk safe from FIQ/NMI context as well. This is not an ARM specific issue. Any architecture which has NMI like facilities has the problem of doing printk from that context. Steven is working on a mitigation for that. https://lkml.org/lkml/2014/11/18/1146
Thanks,
tglx