On mostly-idle systems, we have observed that toggle_allocation_gate() is a cause of frequent wake-ups, preventing an otherwise idle CPU to go into a lower power state.
A late change in KFENCE's development, due to a potential deadlock [1], required changing the scheduling-friendly wait_event_timeout() and wake_up() to an open-coded wait-loop using schedule_timeout(). [1] https://lkml.kernel.org/r/000000000000c0645805b7f98...@google.com To avoid unnecessary wake-ups, switch to using wait_event_timeout(). Unfortunately, we still cannot use a version with direct wake_up() in __kfence_alloc() due to the same potential for deadlock as in [1]. Instead, add a level of indirection via an irq_work that is scheduled if we determine that the kfence_timer requires a wake_up(). Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure") Signed-off-by: Marco Elver <el...@google.com> --- lib/Kconfig.kfence | 1 + mm/kfence/core.c | 58 +++++++++++++++++++++++++++++++++------------- 2 files changed, 43 insertions(+), 16 deletions(-) diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence index 78f50ccb3b45..e641add33947 100644 --- a/lib/Kconfig.kfence +++ b/lib/Kconfig.kfence @@ -7,6 +7,7 @@ menuconfig KFENCE bool "KFENCE: low-overhead sampling-based memory safety error detector" depends on HAVE_ARCH_KFENCE && (SLAB || SLUB) select STACKTRACE + select IRQ_WORK help KFENCE is a low-overhead sampling-based detector of heap out-of-bounds access, use-after-free, and invalid-free errors. KFENCE is designed diff --git a/mm/kfence/core.c b/mm/kfence/core.c index 768dbd58170d..5f0a56041549 100644 --- a/mm/kfence/core.c +++ b/mm/kfence/core.c @@ -10,6 +10,7 @@ #include <linux/atomic.h> #include <linux/bug.h> #include <linux/debugfs.h> +#include <linux/irq_work.h> #include <linux/kcsan-checks.h> #include <linux/kfence.h> #include <linux/kmemleak.h> @@ -587,6 +588,20 @@ late_initcall(kfence_debugfs_init); /* === Allocation Gate Timer ================================================ */ +#ifdef CONFIG_KFENCE_STATIC_KEYS +/* Wait queue to wake up allocation-gate timer task. */ +static DECLARE_WAIT_QUEUE_HEAD(allocation_wait); + +static void wake_up_kfence_timer(struct irq_work *work) +{ + wake_up(&allocation_wait); +} +static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer); + +/* Indicate if timer task is waiting, to avoid unnecessary irq_work. */ +static bool kfence_timer_waiting; +#endif + /* * Set up delayed work, which will enable and disable the static key. We need to * use a work queue (rather than a simple timer), since enabling and disabling a @@ -604,25 +619,16 @@ static void toggle_allocation_gate(struct work_struct *work) if (!READ_ONCE(kfence_enabled)) return; - /* Enable static key, and await allocation to happen. */ atomic_set(&kfence_allocation_gate, 0); #ifdef CONFIG_KFENCE_STATIC_KEYS + /* Enable static key, and await allocation to happen. */ static_branch_enable(&kfence_allocation_key); - /* - * Await an allocation. Timeout after 1 second, in case the kernel stops - * doing allocations, to avoid stalling this worker task for too long. - */ - { - unsigned long end_wait = jiffies + HZ; - - do { - set_current_state(TASK_UNINTERRUPTIBLE); - if (atomic_read(&kfence_allocation_gate) != 0) - break; - schedule_timeout(1); - } while (time_before(jiffies, end_wait)); - __set_current_state(TASK_RUNNING); - } + + WRITE_ONCE(kfence_timer_waiting, true); + smp_mb(); /* See comment in __kfence_alloc(). */ + wait_event_timeout(allocation_wait, atomic_read(&kfence_allocation_gate), HZ); + smp_store_release(&kfence_timer_waiting, false); /* Order after wait_event(). */ + /* Disable static key and reset timer. */ static_branch_disable(&kfence_allocation_key); #endif @@ -729,6 +735,26 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) */ if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1) return NULL; +#ifdef CONFIG_KFENCE_STATIC_KEYS + /* + * Read of kfence_timer_waiting must be ordered after write to + * kfence_allocation_gate (fully ordered per atomic_inc_return()). + * + * Conversely, the write to kfence_timer_waiting must be ordered before + * the check of kfence_allocation_gate in toggle_allocation_gate(). + * + * This ensures that toggle_allocation_gate() always sees the updated + * kfence_allocation_gate, or we see that the timer is waiting and will + * queue the work to wake it up. + */ + if (READ_ONCE(kfence_timer_waiting)) { + /* + * Calling wake_up() here may deadlock when allocations happen + * from within timer code. Use an irq_work to defer it. + */ + irq_work_queue(&wake_up_kfence_timer_work); + } +#endif if (!READ_ONCE(kfence_enabled)) return NULL; -- 2.31.1.368.gbe11c130af-goog
