genirq: Remove unnecessary NULL check of the kstat_irqs field

The kstat_irqs field of struct irq_desc is used to store a per-cpu count
of interrupt events. It is initialized in init_desc(), along with all
the other fields in struct irq_desc that need explicit initialization,
and therefore it's always available (non-NULL) for any valid interrupt
descriptor (with a caveat - see below).

When CONFIG_SPARSE_IRQ is enabled, all interrupt descriptors are always
allocated dynamically via alloc_desc(), which calls init_desc(), so in
that case kstat_irqs is guaranteed to be non-NULL before a valid struct
irq_desc pointer is even returned.

By contrast, when CONFIG_SPARSE_IRQ is disabled, interrupt descriptors
are allocated statically in the irq_desc[] array, and kstat_irqs is
initialized implicitly to NULL. The per-cpu pointer is initialized only
later, for all descriptors, via start_kernel() -> early_irq_init() ->
init_desc().

The kstat_irqs field is used mostly for printing interrupt statistics
(i.e. reading /proc/interrupts), and that cannot happen until much
later, when user-space is fully initialized. So, there is no concern
with that use case.

The list below includes all functions where the NULL check is removed,
along with a list of all possible call chains and/or a brief explanation
of why it's safe to remove the NULL check in that case.

  * irq_desc_kstat_cpu() [include/linux/irqdesc.h]
    - All direct call sites use it for printing IRQ statistics.

    - Indirect call site: per_cpu_count_show() - also used for printing
      interruptstatistics.

  * kstat_irqs_cpu() [kernel/irq/irqdesc.c]
    - Called by sun3_int7() and sun3_int5() [arch/m68k/sun3/sun3ints.c]
      These are interrupt handlers and cannot be called until their
      corresponding interrupts are initialized in sun3_init_IRQ(). The
      call chain leading to that is:
        start_kernel() -> init_IRQ() [arch/m68k/kernel/ints.c] ->
        mach_init_IRQ = sun3_init_IRQ()
      The init_IRQ() call happens right *after* the early_irq_init() call,
      which means the descriptors are already fully initialized by the
      time the interrupt handlers are even registered.

    - Called by show_interrupts() [arch/s390/kernel/irq.c] - used for
      printing interrupt statistics.

  * kstat_irqs() [kernel/irq/irqdesc.c]
      The only possible call chain is via fs/proc/stat.c:
      stat_open() -> show_stat() -> show_all_irqs() -> kstat_irqs_usr() ->
      kstat_irqs()
      It is used for printing interrupt statistics.

  * kstat_snapshot_irqs()
      The only possible call chain is via kernel/watchdog.c:
        watchdog_timer_fn() -> is_softlockup() -> start_counting_irqs() ->
        kstat_snapshot_irqs()
      The watchdog timer cannot fire early, before early_irq_init().

  * kstat_get_irq_since_snapshot()
      The only possible call chain is via kernel/watchdog.c:
        watchdog_timer_fn() -> report_cpu_status() -> print_irq_counts() ->
        kstat_get_irq_since_snapshot()
      The watchdog timer cannot fire early, before early_irq_init().

Signed-off-by: Radu Rendec <radu@rendec.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260705210951.2717741-2-radu@rendec.net

0 files changed, 0 insertions, 0 deletions