[tip: locking/urgent] x86/split_lock: Simplify reenabling

From: tip-bot2 for Maksim Davydov
Date: Tue Mar 25 2025 - 05:35:29 EST


The following commit has been merged into the locking/urgent branch of tip:

Commit-ID: 0e1ff67d164be45e8ddfea5aaf5803224ede0805
Gitweb: https://git.kernel.org/tip/0e1ff67d164be45e8ddfea5aaf5803224ede0805
Author: Maksim Davydov <davydov-max@xxxxxxxxxxxxxx>
AuthorDate: Tue, 25 Mar 2025 11:58:07 +03:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 25 Mar 2025 10:16:50 +01:00

x86/split_lock: Simplify reenabling

When split_lock_mitigate is disabled, each CPU needs its own delayed_work
structure. They are used to reenable split lock detection after its
disabling. But delayed_work structure must be correctly initialized after
its allocation.

Current implementation uses deferred initialization that makes the
split lock handler code unclear. The code can be simplified a bit
if the initialization is moved to the appropriate initcall.

sld_setup() is called before setup_per_cpu_areas(), thus it can't be used
for this purpose, so introduce an independent initcall for
the initialization.

[ mingo: Simplified the 'work' assignment line a bit more. ]

Signed-off-by: Maksim Davydov <davydov-max@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250325085807.171885-1-davydov-max@xxxxxxxxxxxxxx
---
arch/x86/kernel/cpu/bus_lock.c | 35 ++++++++++++++++++++++-----------
1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
index 97222ef..237faf7 100644
--- a/arch/x86/kernel/cpu/bus_lock.c
+++ b/arch/x86/kernel/cpu/bus_lock.c
@@ -201,6 +201,26 @@ static void __split_lock_reenable(struct work_struct *work)
static DEFINE_PER_CPU(struct delayed_work, sl_reenable);

/*
+ * Per-CPU delayed_work can't be statically initialized properly because
+ * the struct address is unknown. Thus per-CPU delayed_work structures
+ * have to be initialized during kernel initialization and after calling
+ * setup_per_cpu_areas().
+ */
+static int __init setup_split_lock_delayed_work(void)
+{
+ unsigned int cpu;
+
+ for_each_possible_cpu(cpu) {
+ struct delayed_work *work = per_cpu_ptr(&sl_reenable, cpu);
+
+ INIT_DELAYED_WORK(work, __split_lock_reenable);
+ }
+
+ return 0;
+}
+pure_initcall(setup_split_lock_delayed_work);
+
+/*
* If a CPU goes offline with pending delayed work to re-enable split lock
* detection then the delayed work will be executed on some other CPU. That
* handles releasing the buslock_sem, but because it executes on a
@@ -219,15 +239,16 @@ static int splitlock_cpu_offline(unsigned int cpu)

static void split_lock_warn(unsigned long ip)
{
- struct delayed_work *work = NULL;
+ struct delayed_work *work;
int cpu;
+ unsigned int saved_sld_mitigate = READ_ONCE(sysctl_sld_mitigate);

if (!current->reported_split_lock)
pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
current->comm, current->pid, ip);
current->reported_split_lock = 1;

- if (sysctl_sld_mitigate) {
+ if (saved_sld_mitigate) {
/*
* misery factor #1:
* sleep 10ms before trying to execute split lock.
@@ -240,18 +261,10 @@ static void split_lock_warn(unsigned long ip)
*/
if (down_interruptible(&buslock_sem) == -EINTR)
return;
- work = &sl_reenable_unlock;
}

cpu = get_cpu();
-
- if (!work) {
- work = this_cpu_ptr(&sl_reenable);
- /* Deferred initialization of per-CPU struct */
- if (!work->work.func)
- INIT_DELAYED_WORK(work, __split_lock_reenable);
- }
-
+ work = saved_sld_mitigate ? &sl_reenable_unlock : per_cpu_ptr(&sl_reenable, cpu);
schedule_delayed_work_on(cpu, work, 2);

/* Disable split lock detection on this CPU to make progress */