[PATCH printk v5 22/30] printk: nbcon: Add unsafe flushing on panic

John Ogness
2024-05-02T21:38:54Z

Add nbcon_atomic_flush_unsafe() to flush all nbcon consoles
using the write_atomic() callback and allowing unsafe hostile
takeovers. Call this at the end of panic() as a final attempt
to flush any pending messages.

Note that legacy consoles use unsafe methods for flushing
from the beginning of panic (see bust_spinlocks()). Therefore,
systems using both legacy and nbcon consoles may still fail to
see panic messages due to unsafe legacy console usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
include/linux/printk.h | 5 +++++
kernel/panic.c | 1 +
kernel/printk/nbcon.c | 32 +++++++++++++++++++++++++-------
3 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index d0a1106388d1..439decc792b6 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -197,6 +197,7 @@ extern asmlinkage void dump_stack(void) __cold;
void printk_trigger_flush(void);
extern bool nbcon_driver_try_acquire(struct console *con);
extern void nbcon_driver_release(struct console *con);
+void nbcon_atomic_flush_unsafe(void);
#else
static inline __printf(1, 0)
int vprintk(const char *s, va_list args)
@@ -286,6 +287,10 @@ static inline void nbcon_driver_release(struct console *con)
{
}

+static inline void nbcon_atomic_flush_unsafe(void)
+{
+}
+
#endif

bool this_cpu_in_panic(void);
diff --git a/kernel/panic.c b/kernel/panic.c
index f22d8f33ea14..c039f8e1ddae 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -453,6 +453,7 @@ void panic(const char *fmt, ...)
* Explicitly flush the kernel log buffer one last time.
*/
console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+ nbcon_atomic_flush_unsafe();

local_irq_enable();
for (i = 0; ; i += PANIC_TIMER_STEP) {
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index abd3835a3466..8f6b3858ab27 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1044,6 +1044,7 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
* write_atomic() callback
* @con: The nbcon console to flush
* @stop_seq: Flush up until this record
+ * @allow_unsafe_takeover: True, to allow unsafe hostile takeovers
*
* Return: 0 if @con was flushed up to @stop_seq Otherwise, error code on
* failure.
@@ -1062,7 +1063,8 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
* returned, it cannot be expected that the unfinalized record will become
* available.
*/
-static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq,
+ bool allow_unsafe_takeover)
{
struct nbcon_write_context wctxt = { };
struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
@@ -1071,6 +1073,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
ctxt->console = con;
ctxt->spinwait_max_us = 2000;
ctxt->prio = nbcon_get_default_prio();
+ ctxt->allow_unsafe_takeover = allow_unsafe_takeover;

if (!nbcon_context_try_acquire(ctxt))
return -EPERM;
@@ -1100,13 +1103,15 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
* write_atomic() callback
* @con: The nbcon console to flush
* @stop_seq: Flush up until this record
+ * @allow_unsafe_takeover: True, to allow unsafe hostile takeovers
*
* This will stop flushing before @stop_seq if another context has ownership.
* That context is then responsible for the flushing. Likewise, if new records
* are added while this context was flushing and there is no other context
* to handle the printing, this context must also flush those records.
*/
-static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq,
+ bool allow_unsafe_takeover)
{
unsigned long flags;
int err;
@@ -1120,7 +1125,7 @@ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
*/
local_irq_save(flags);

- err = __nbcon_atomic_flush_pending_con(con, stop_seq);
+ err = __nbcon_atomic_flush_pending_con(con, stop_seq, allow_unsafe_takeover);

local_irq_restore(flags);

@@ -1144,8 +1149,9 @@ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
* __nbcon_atomic_flush_pending - Flush all nbcon consoles using their
* write_atomic() callback
* @stop_seq: Flush up until this record
+ * @allow_unsafe_takeover: True, to allow unsafe hostile takeovers
*/
-static void __nbcon_atomic_flush_pending(u64 stop_seq)
+static void __nbcon_atomic_flush_pending(u64 stop_seq, bool allow_unsafe_takeover)
{
struct console *con;
int cookie;
@@ -1163,7 +1169,7 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
if (nbcon_seq_read(con) >= stop_seq)
continue;

- nbcon_atomic_flush_pending_con(con, stop_seq);
+ nbcon_atomic_flush_pending_con(con, stop_seq, allow_unsafe_takeover);
}
console_srcu_read_unlock(cookie);
}
@@ -1178,7 +1184,19 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
*/
void nbcon_atomic_flush_pending(void)
{
- __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+ __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), false);
+}
+
+/**
+ * nbcon_atomic_flush_unsafe - Flush all nbcon consoles using their
+ * write_atomic() callback and allowing unsafe hostile takeovers
+ *
+ * Flush the backlog up through the currently newest record. Unsafe hostile
+ * takeovers will be performed, if necessary.
+ */
+void nbcon_atomic_flush_unsafe(void)
+{
+ __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true);
}

/**
@@ -1307,7 +1325,7 @@ void nbcon_driver_release(struct console *con)
cookie = console_srcu_read_lock();
if (console_is_usable(con, console_srcu_read_flags(con)) &&
prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
- __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
+ __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false);
}
console_srcu_read_unlock(cookie);
}
--
2.39.2