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 <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-27-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9687089f5acebda3ab9b20ec88f21b6ae947d197..2e083f01f8a3b391c6c865db6ab11d3e7aee395e 100644 (file)
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -202,6 +202,7 @@ void printk_trigger_flush(void);
 void console_try_replay_all(void);
 extern bool nbcon_device_try_acquire(struct console *con);
 extern void nbcon_device_release(struct console *con);
+void nbcon_atomic_flush_unsafe(void);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -294,6 +295,10 @@ static inline void nbcon_device_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 2a0449144f82ea8c3daef723b56f63a22e128657..df37c913b0101fc1b245014e3db40dfa27c1aa32 100644 (file)
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -463,6 +463,7 @@ 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 269aeed1806493aca68a61b8c303696e0211ce27..afdb16c1c733747089afa88936ffc3530514b5e1 100644 (file)
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1083,6 +1083,7 @@ 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.
@@ -1101,7 +1102,8 @@ 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);
@@ -1110,6 +1112,7 @@ 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;
@@ -1140,13 +1143,15 @@ 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;
@@ -1160,7 +1165,7 @@ again:
         */
        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);
 
@@ -1190,8 +1195,9 @@ again:
  * __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;
@@ -1209,7 +1215,7 @@ 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);
 }
@@ -1225,7 +1231,19 @@ 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);
 }
 
 /**
@@ -1342,7 +1360,7 @@ void nbcon_device_release(struct console *con)
        if (console_is_usable(con, console_srcu_read_flags(con)) &&
            prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
                if (!have_boot_console) {
-                       __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
+                       __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false);
                } else if (!is_printk_legacy_deferred()) {
                        if (console_trylock())
                                console_unlock();