2024-02-18 19:01:18

by John Ogness

Subject: [PATCH printk v2 18/26] printk: nbcon: Add unsafe flushing on panic

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 <[email protected]>
---
include/linux/printk.h | 5 +++++
kernel/panic.c | 1 +
kernel/printk/nbcon.c | 18 ++++++++++++++++--
3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index ef57a4d93ae2..fe37395af9e2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -199,6 +199,7 @@ extern asmlinkage void dump_stack(void) __cold;
void printk_trigger_flush(void);
extern void uart_nbcon_acquire(struct uart_port *up);
extern void uart_nbcon_release(struct uart_port *up);
+void nbcon_atomic_flush_unsafe(void);
#else
static inline __printf(1, 0)
int vprintk(const char *s, va_list args)
@@ -287,6 +288,10 @@ static inline void uart_nbcon_release(struct uart_port *up)
{
}

+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 1a18549e43b8..64dedd79e880 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1025,8 +1025,9 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
* __nbcon_atomic_flush_all - 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_all(u64 stop_seq)
+static void __nbcon_atomic_flush_all(u64 stop_seq, bool allow_unsafe_takeover)
{
struct nbcon_write_context wctxt = { };
struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
@@ -1054,6 +1055,7 @@ static void __nbcon_atomic_flush_all(u64 stop_seq)
memset(ctxt, 0, sizeof(*ctxt));
ctxt->console = con;
ctxt->spinwait_max_us = 2000;
+ ctxt->allow_unsafe_takeover = allow_unsafe_takeover;

/*
* Atomic flushing does not use console driver
@@ -1088,7 +1090,19 @@ static void __nbcon_atomic_flush_all(u64 stop_seq)
*/
void nbcon_atomic_flush_all(void)
{
- __nbcon_atomic_flush_all(prb_next_reserve_seq(prb));
+ __nbcon_atomic_flush_all(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_all(prb_next_reserve_seq(prb), true);
}

/**
--
2.39.2

2024-02-29 13:53:48

by Petr Mladek

Subject: Re: [PATCH printk v2 18/26] printk: nbcon: Add unsafe flushing on panic

On Sun 2024-02-18 20:03:18, John Ogness wrote:
> 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 <[email protected]>

Reviewed-by: Petr Mladek <[email protected]>

Best Regards,
Petr