linux-kernel - [tip: locking/urgent] lockdep: Fix wait context check on softirq for PREEMPT

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Message-ID: <174289718274.14745.16349720260441360293.tip-bot2@tip-bot2>
Date: Tue, 25 Mar 2025 10:06:21 -0000
From: "tip-bot2 for Ryo Takakura" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Ryo Takakura <ryotkkr98@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Ingo Molnar <mingo@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: locking/urgent] lockdep: Fix wait context check on softirq for
 PREEMPT_RT

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

Commit-ID:     61c39d8c83e2077f33e0a2c8980a76a7f323f0ce
Gitweb:        https://git.kernel.org/tip/61c39d8c83e2077f33e0a2c8980a76a7f323f0ce
Author:        Ryo Takakura <ryotkkr98@...il.com>
AuthorDate:    Fri, 21 Mar 2025 07:33:22 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 25 Mar 2025 10:46:44 +01:00

lockdep: Fix wait context check on softirq for PREEMPT_RT

Since:

  0c1d7a2c2d32 ("lockdep: Remove softirq accounting on PREEMPT_RT.")

the wait context test for mutex usage within "in softirq context" fails
as it references @softirq_context:

    | wait context tests |
    --------------------------------------------------------------------------
                                   | rcu  | raw  | spin |mutex |
    --------------------------------------------------------------------------
                 in hardirq context:  ok  |  ok  |  ok  |  ok  |
  in hardirq context (not threaded):  ok  |  ok  |  ok  |  ok  |
                 in softirq context:  ok  |  ok  |  ok  |FAILED|

As a fix, add lockdep map for BH disabled section. This fixes the
issue by letting us catch cases when local_bh_disable() gets called
with preemption disabled where local_lock doesn't get acquired.
In the case of "in softirq context" selftest, local_bh_disable() was
being called with preemption disable as it's early in the boot.

[ boqun: Move the lockdep annotations into __local_bh_*() to avoid false
         positives because of unpaired local_bh_disable() reported by
	 Borislav Petkov and Peter Zijlstra, and make bh_lock_map
	 only exist for PREEMPT_RT. ]

[ mingo: Restored authorship and improved the bh_lock_map definition. ]

Signed-off-by: Ryo Takakura <ryotkkr98@...il.com>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20250321143322.79651-1-boqun.feng@gmail.com
---
 kernel/softirq.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 4dae6ac..513b194 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -126,6 +126,18 @@ static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
 	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
 };
 
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+static struct lock_class_key bh_lock_key;
+struct lockdep_map bh_lock_map = {
+	.name			= "local_bh",
+	.key			= &bh_lock_key,
+	.wait_type_outer	= LD_WAIT_FREE,
+	.wait_type_inner	= LD_WAIT_CONFIG, /* PREEMPT_RT makes BH preemptible. */
+	.lock_type		= LD_LOCK_PERCPU,
+};
+EXPORT_SYMBOL_GPL(bh_lock_map);
+#endif
+
 /**
  * local_bh_blocked() - Check for idle whether BH processing is blocked
  *
@@ -148,6 +160,8 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 
 	WARN_ON_ONCE(in_hardirq());
 
+	lock_map_acquire_read(&bh_lock_map);
+
 	/* First entry of a task into a BH disabled section? */
 	if (!current->softirq_disable_cnt) {
 		if (preemptible()) {
@@ -211,6 +225,8 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	WARN_ON_ONCE(in_hardirq());
 	lockdep_assert_irqs_enabled();
 
+	lock_map_release(&bh_lock_map);
+
 	local_irq_save(flags);
 	curcnt = __this_cpu_read(softirq_ctrl.cnt);
 
@@ -261,6 +277,8 @@ static inline void ksoftirqd_run_begin(void)
 /* Counterpart to ksoftirqd_run_begin() */
 static inline void ksoftirqd_run_end(void)
 {
+	/* pairs with the lock_map_acquire_read() in ksoftirqd_run_begin() */
+	lock_map_release(&bh_lock_map);
 	__local_bh_enable(SOFTIRQ_OFFSET, true);
 	WARN_ON_ONCE(in_interrupt());
 	local_irq_enable();