On Wed, Feb 13, 2019 at 5:05 AM Frederic Weisbecker <frede...@kernel.org> wrote: > On Tue, Feb 05, 2019 at 07:34:31PM +0100, Denys Vlasenko wrote: > > SOFTIRQ is a counter. > > Why here: > > > > #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET) > > #define in_task() (!(preempt_count() & \ > > (NMI_MASK | HARDIRQ_MASK | > > SOFTIRQ_OFFSET))) > > > > we check only lowest bit? > > So we have SOFTIRQ_OFFSET that is used when serving softirqs. > And we have SOFTIRQ_DISABLE_OFFSET that is used when we disable > softirqs. > > I think the choice is right on both tests above, or am I missing something?
>From my reading of the following: * We put the hardirq and softirq counter into the preemption * counter. The bitmask has the following meaning: * - bits 0-7 are the preemption count (max preemption depth: 256) * - bits 8-15 are the softirq count (max # of softirqs: 256) * The hardirq count could in theory be the same as the number of * interrupts in the system, but we run all interrupt handlers with * interrupts disabled, so we cannot have nesting interrupts. Though * there are a few palaeontologic drivers which reenable interrupts in * the handler, so we need more than one bit here. * PREEMPT_MASK: 0x000000ff * SOFTIRQ_MASK: 0x0000ff00 * HARDIRQ_MASK: 0x000f0000 * NMI_MASK: 0x00100000 * PREEMPT_NEED_RESCHED: 0x80000000 */ #define PREEMPT_BITS 8 #define SOFTIRQ_BITS 8 #define HARDIRQ_BITS 4 #define NMI_BITS 1 it seems that 8-bit SOFTIRQ_MASK contains depth count of nested softirqs. Therefore, to test whether we are in softirq, you need: define in_serving_softirq() (softirq_count() & SOFTIRQ_MASK) But existing code uses SOFTIRQ_OFFSET, not SOFTIRQ_MASK, for the mask. This means that if nest count is e.g. 2, in_serving_softirq() will return "false".
