• Mark Rutland's avatar
    arm64/cpufeature: don't use mutex in bringup path · 63a1e1c9
    Mark Rutland authored
    
    
    Currently, cpus_set_cap() calls static_branch_enable_cpuslocked(), which
    must take the jump_label mutex.
    
    We call cpus_set_cap() in the secondary bringup path, from the idle
    thread where interrupts are disabled. Taking a mutex in this path "is a
    NONO" regardless of whether it's contended, and something we must avoid.
    We didn't spot this until recently, as ___might_sleep() won't warn for
    this case until all CPUs have been brought up.
    
    This patch avoids taking the mutex in the secondary bringup path. The
    poking of static keys is deferred until enable_cpu_capabilities(), which
    runs in a suitable context on the boot CPU. To account for the static
    keys being set later, cpus_have_const_cap() is updated to use another
    static key to check whether the const cap keys have been initialised,
    falling back to the caps bitmap until this is the case.
    
    This means that users of cpus_have_const_cap() gain should only gain a
    single additional NOP in the fast path once the const caps are
    initialised, but should always see the current cap value.
    
    The hyp code should never dereference the caps array, since the caps are
    initialized before we run the module initcall to initialise hyp. A check
    is added to the hyp init code to document this requirement.
    
    This change will sidestep a number of issues when the upcoming hotplug
    locking rework is merged.
    
    Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
    Reviewed-by: default avatarMarc Zyniger <marc.zyngier@arm.com>
    Reviewed-by: default avatarSuzuki Poulose <suzuki.poulose@arm.com>
    Acked-by: default avatarWill Deacon <will.deacon@arm.com>
    Cc: Christoffer Dall <christoffer.dall@linaro.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sebastian Sewior <bigeasy@linutronix.de>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    63a1e1c9