Legacy console printing from printk() caller context may invoke
the console driver from atomic context. This leads to a lockdep
splat because the console driver will acquire a sleeping lock
and the caller may already hold a spinning lock. This is noticed
by lockdep on !PREEMPT_RT configurations because it will lead to
a problem on PREEMPT_RT.
However, on PREEMPT_RT the printing path from atomic context is
always avoided and the console driver is always invoked from a
dedicated thread. Thus the lockdep splat on !PREEMPT_RT is a
false positive.
For !PREEMPT_RT override the lock-context before invoking the
console driver to avoid the false positive.
Do not override the lock-context for PREEMPT_RT in order to
allow lockdep to catch any real locking context issues related
to the write callback usage.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
kernel/printk/printk.c | 83 ++++++++++++++++++++++++++++++++----------
1 file changed, 63 insertions(+), 20 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c4a6aaddf8e6..19e998c4985b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2981,6 +2981,34 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
return true;
}
+/*
+ * Legacy console printing from printk() caller context does not respect
+ * raw_spinlock/spinlock nesting. For !PREEMPT_RT the lockdep warning is a
+ * false positive. For PREEMPT_RT the false positive condition does not
+ * occur.
+ *
+ * This map is used to temporarily establish LD_WAIT_SLEEP context for the
+ * console write() callback when legacy printing to avoid false positive
+ * lockdep complaints, thus allowing lockdep to continue to function for
+ * real issues.
+ */
+#ifdef CONFIG_PREEMPT_RT
+static inline void printk_legacy_allow_spinlock_enter(void) { }
+static inline void printk_legacy_allow_spinlock_exit(void) { }
+#else
+static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);
+
+static inline void printk_legacy_allow_spinlock_enter(void)
+{
+ lock_map_acquire_try(&printk_legacy_map);
+}
+
+static inline void printk_legacy_allow_spinlock_exit(void)
+{
+ lock_map_release(&printk_legacy_map);
+}
+#endif /* CONFIG_PREEMPT_RT */
+
/*
* Used as the printk buffers for non-panic, serialized console printing.
* This is for legacy (!CON_NBCON) as well as all boot (CON_BOOT) consoles.
@@ -3030,31 +3058,46 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
con->dropped = 0;
}
- /*
- * While actively printing out messages, if another printk()
- * were to occur on another CPU, it may wait for this one to
- * finish. This task can not be preempted if there is a
- * waiter waiting to take over.
- *
- * Interrupts are disabled because the hand over to a waiter
- * must not be interrupted until the hand over is completed
- * (@console_waiter is cleared).
- */
- printk_safe_enter_irqsave(flags);
- console_lock_spinning_enable();
+ /* Write everything out to the hardware. */
- /* Do not trace print latency. */
- stop_critical_timings();
+ if (force_legacy_kthread() && !panic_in_progress()) {
+ /*
+ * With forced threading this function is in a task context
+ * (either legacy kthread or get_init_console_seq()). There
+ * is no need for concern about printk reentrance, handovers,
+ * or lockdep complaints.
+ */
- /* Write everything out to the hardware. */
- con->write(con, outbuf, pmsg.outbuf_len);
+ con->write(con, outbuf, pmsg.outbuf_len);
+ con->seq = pmsg.seq + 1;
+ } else {
+ /*
+ * While actively printing out messages, if another printk()
+ * were to occur on another CPU, it may wait for this one to
+ * finish. This task can not be preempted if there is a
+ * waiter waiting to take over.
+ *
+ * Interrupts are disabled because the hand over to a waiter
+ * must not be interrupted until the hand over is completed
+ * (@console_waiter is cleared).
+ */
+ printk_safe_enter_irqsave(flags);
+ console_lock_spinning_enable();
- start_critical_timings();
+ /* Do not trace print latency. */
+ stop_critical_timings();
- con->seq = pmsg.seq + 1;
+ printk_legacy_allow_spinlock_enter();
+ con->write(con, outbuf, pmsg.outbuf_len);
+ printk_legacy_allow_spinlock_exit();
- *handover = console_lock_spinning_disable_and_check(cookie);
- printk_safe_exit_irqrestore(flags);
+ start_critical_timings();
+
+ con->seq = pmsg.seq + 1;
+
+ *handover = console_lock_spinning_disable_and_check(cookie);
+ printk_safe_exit_irqrestore(flags);
+ }
skip:
return true;
}
--
2.39.2