From 676e8cf70cb0533e1118e29898c9a9c33ae3a10f Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 9 May 2025 13:36:59 +0200 Subject: sched,livepatch: Untangle cond_resched() and live-patching With the goal of deprecating / removing VOLUNTARY preempt, live-patch needs to stop relying on cond_resched() to make forward progress. Instead, rely on schedule() with TASK_FREEZABLE set. Just like live-patching, the freezer needs to be able to stop tasks in a safe / known state. [bigeasy: use likely() in __klp_sched_try_switch() and update comments] Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Thomas Gleixner Reviewed-by: Petr Mladek Tested-by: Petr Mladek Tested-by: Miroslav Benes Acked-by: Miroslav Benes Acked-by: Josh Poimboeuf Link: https://lore.kernel.org/r/20250509113659.wkP_HJ5z@linutronix.de --- kernel/livepatch/transition.c | 49 +++++++++++++------------------------------ 1 file changed, 14 insertions(+), 35 deletions(-) (limited to 'kernel/livepatch') diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c index ba069459c101..2351a19ac2a9 100644 --- a/kernel/livepatch/transition.c +++ b/kernel/livepatch/transition.c @@ -29,22 +29,13 @@ static unsigned int klp_signals_cnt; /* * When a livepatch is in progress, enable klp stack checking in - * cond_resched(). This helps CPU-bound kthreads get patched. + * schedule(). This helps CPU-bound kthreads get patched. */ -#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL) - -#define klp_cond_resched_enable() sched_dynamic_klp_enable() -#define klp_cond_resched_disable() sched_dynamic_klp_disable() - -#else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */ DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key); -EXPORT_SYMBOL(klp_sched_try_switch_key); -#define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key) -#define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key) - -#endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */ +#define klp_resched_enable() static_branch_enable(&klp_sched_try_switch_key) +#define klp_resched_disable() static_branch_disable(&klp_sched_try_switch_key) /* * This work can be performed periodically to finish patching or unpatching any @@ -365,26 +356,18 @@ static bool klp_try_switch_task(struct task_struct *task) void __klp_sched_try_switch(void) { - if (likely(!klp_patch_pending(current))) - return; - /* - * This function is called from cond_resched() which is called in many - * places throughout the kernel. Using the klp_mutex here might - * deadlock. - * - * Instead, disable preemption to prevent racing with other callers of - * klp_try_switch_task(). Thanks to task_call_func() they won't be - * able to switch this task while it's running. + * This function is called from __schedule() while a context switch is + * about to happen. Preemption is already disabled and klp_mutex + * can't be acquired. + * Disabled preemption is used to prevent racing with other callers of + * klp_try_switch_task(). Thanks to task_call_func() they won't be + * able to switch to this task while it's running. */ - preempt_disable(); + lockdep_assert_preemption_disabled(); - /* - * Make sure current didn't get patched between the above check and - * preempt_disable(). - */ - if (unlikely(!klp_patch_pending(current))) - goto out; + if (likely(!klp_patch_pending(current))) + return; /* * Enforce the order of the TIF_PATCH_PENDING read above and the @@ -395,11 +378,7 @@ void __klp_sched_try_switch(void) smp_rmb(); klp_try_switch_task(current); - -out: - preempt_enable(); } -EXPORT_SYMBOL(__klp_sched_try_switch); /* * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set. @@ -508,7 +487,7 @@ void klp_try_complete_transition(void) } /* Done! Now cleanup the data structures. */ - klp_cond_resched_disable(); + klp_resched_disable(); patch = klp_transition_patch; klp_complete_transition(); @@ -560,7 +539,7 @@ void klp_start_transition(void) set_tsk_thread_flag(task, TIF_PATCH_PENDING); } - klp_cond_resched_enable(); + klp_resched_enable(); klp_signals_cnt = 0; } -- cgit v1.2.3-59-g8ed1b