printk: nbcon: Show replay message on takeover

An emergency or panic context can takeover console ownership
while the current owner was printing a printk message. The
atomic printer will re-print the message that the previous
owner was printing. However, this can look confusing to the
user and may even seem as though a message was lost.

  [3430014.1
  [3430014.181123] usb 1-2: Product: USB Audio

Add a new field @nbcon_prev_seq to struct console to track
the sequence number to print that was assigned to the previous
console owner. If this matches the sequence number to print
that the current owner is assigned, then a takeover must have
occurred. In this case, print an additional message to inform
the user that the previous message is being printed again.

  [3430014.1
  ** replaying previous printk message **
  [3430014.181123] usb 1-2: Product: USB Audio

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-12-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/include/linux/console.h b/include/linux/console.h
index 788ce9c829f64248f8f8d3d95727029dc93abed9..eba367bf605da6ef798ea0b6587b7c2f2ad4188d 100644 (file)
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -325,6 +325,7 @@ struct nbcon_write_context {
  * @nbcon_state:       State for nbcon consoles
  * @nbcon_seq:         Sequence number of the next record for nbcon to print
  * @nbcon_device_ctxt: Context available for non-printing operations
+ * @nbcon_prev_seq:    Seq num the previous nbcon owner was assigned to print
  * @pbufs:             Pointer to nbcon private buffer
  * @kthread:           Printer kthread for this console
  * @rcuwait:           RCU-safe wait object for @kthread waking
@@ -459,6 +460,7 @@ struct console {
        atomic_t                __private nbcon_state;
        atomic_long_t           __private nbcon_seq;
        struct nbcon_context    __private nbcon_device_ctxt;
+       atomic_long_t           __private nbcon_prev_seq;
 
        struct printk_buffers   *pbufs;
        struct task_struct      *kthread;

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 8166e24f8780c2dbf9e1b2b96f517c69e1b92a2d..c365d25b13c78bfcfc48fa827d40d2c3838c2d03 100644 (file)
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -319,4 +319,5 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 
 #ifdef CONFIG_PRINTK
 void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);
+void console_prepend_replay(struct printk_message *pmsg);
 #endif

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 5146ce9853a350b64e831d1f239939323e8a4ec5..98440889d222d94289c52fe8b06e16dfdad135c3 100644 (file)
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -946,7 +946,9 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
                .pbufs = ctxt->pbufs,
        };
        unsigned long con_dropped;
+       struct nbcon_state cur;
        unsigned long dropped;
+       unsigned long ulseq;
 
        /*
         * This function should never be called for consoles that have not
@@ -987,6 +989,29 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
        if (dropped && !is_extended)
                console_prepend_dropped(&pmsg, dropped);
 
+       /*
+        * If the previous owner was assigned the same record, this context
+        * has taken over ownership and is replaying the record. Prepend a
+        * message to let the user know the record is replayed.
+        */
+       ulseq = atomic_long_read(&ACCESS_PRIVATE(con, nbcon_prev_seq));
+       if (__ulseq_to_u64seq(prb, ulseq) == pmsg.seq) {
+               console_prepend_replay(&pmsg);
+       } else {
+               /*
+                * Ensure this context is still the owner before trying to
+                * update @nbcon_prev_seq. Otherwise the value in @ulseq may
+                * not be from the previous owner and instead be some later
+                * value from the context that took over ownership.
+                */
+               nbcon_state_read(con, &cur);
+               if (!nbcon_context_can_proceed(ctxt, &cur))
+                       return false;
+
+               atomic_long_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_prev_seq), &ulseq,
+                                       __u64seq_to_ulseq(pmsg.seq));
+       }
+
        if (!nbcon_context_exit_unsafe(ctxt))
                return false;
 
@@ -1646,6 +1671,7 @@ bool nbcon_alloc(struct console *con)
 
        rcuwait_init(&con->rcuwait);
        init_irq_work(&con->irq_work, nbcon_irq_work);
+       atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);
        nbcon_state_set(con, &state);
 
        /*

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e9458569bcfd6113f22024c78958578c8ba01235..c27dc54f41512b16fd30e8b43ef6e088875377e9 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2902,6 +2902,17 @@ void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
        console_prepend_message(pmsg, "** %lu printk messages dropped **\n", dropped);
 }
 
+/*
+ * Prepend the message in @pmsg->pbufs->outbuf with a "replay message".
+ * @pmsg->outbuf_len is updated appropriately.
+ *
+ * @pmsg is the printk message to prepend.
+ */
+void console_prepend_replay(struct printk_message *pmsg)
+{
+       console_prepend_message(pmsg, "** replaying previous printk message **\n");
+}
+
 /*
  * Read and format the specified record (or a later record if the specified
  * record is not available).