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sched,livepatch: Untangle cond_resched() and live-patching

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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) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Tested-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20250509113659.wkP_HJ5z@linutronix.de
This commit is contained in:
Peter Zijlstra
2025-05-09 13:36:59 +02:00
parent b7ca5743a2
commit 676e8cf70c
4 changed files with 28 additions and 93 deletions
Download Patch File Download Diff File Expand all files Collapse all files

51
kernel/livepatch/transition.c
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@@ -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
@@ -365,27 +356,19 @@ static bool klp_try_switch_task(struct task_struct *task)
void __klp_sched_try_switch(void)
{
/*
* 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.
*/
lockdep_assert_preemption_disabled();
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.
*/
preempt_disable();
/*
* Make sure current didn't get patched between the above check and
* preempt_disable().
*/
if (unlikely(!klp_patch_pending(current)))
goto out;
/*
* Enforce the order of the TIF_PATCH_PENDING read above and the
* klp_target_state read in klp_try_switch_task(). The corresponding
@@ -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;
}