[PATCH AUTOSEL 5.16 33/59] printk: Drop console_sem during panic

Sasha Levin
2022-03-30T11:58:35Z

From: Stephen Brennan <stephen.s.brennan@oracle.com>

[ Upstream commit 8ebc476fd51e6c0fd3174ec1959a20ba99d4c5e5 ]

If another CPU is in panic, we are about to be halted. Try to gracefully
abandon the console_sem, leaving it free for the panic CPU to grab.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220202171821.179394-5-stephen.s.brennan@oracle.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/printk/printk.c | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7c4c53a2c35..92eb7785e6e7 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2588,6 +2588,25 @@ static int have_callable_console(void)
return 0;
}

+/*
+ * Return true when this CPU should unlock console_sem without pushing all
+ * messages to the console. This reduces the chance that the console is
+ * locked when the panic CPU tries to use it.
+ */
+static bool abandon_console_lock_in_panic(void)
+{
+ if (!panic_in_progress())
+ return false;
+
+ /*
+ * We can use raw_smp_processor_id() here because it is impossible for
+ * the task to be migrated to the panic_cpu, or away from it. If
+ * panic_cpu has already been set, and we're not currently executing on
+ * that CPU, then we never will be.
+ */
+ return atomic_read(&panic_cpu) != raw_smp_processor_id();
+}
+
/*
* Can we actually use the console at this time on this cpu?
*
@@ -2736,6 +2755,10 @@ void console_unlock(void)
if (handover)
return;

+ /* Allow panic_cpu to take over the consoles safely */
+ if (abandon_console_lock_in_panic())
+ break;
+
if (do_cond_resched)
cond_resched();
}
@@ -2753,7 +2776,7 @@ void console_unlock(void)
* flush, no worries.
*/
retry = prb_read_valid(prb, next_seq, NULL);
- if (retry && console_trylock())
+ if (retry && !abandon_console_lock_in_panic() && console_trylock())
goto again;
}
EXPORT_SYMBOL(console_unlock);
--
2.34.1