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Message-Id: <20240402221129.2613843-16-john.ogness@linutronix.de>
Date: Wed, 3 Apr 2024 00:17:17 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org
Subject: [PATCH printk v4 15/27] printk: nbcon: Provide function to flush using write_atomic()
From: Thomas Gleixner <tglx@...utronix.de>
Provide nbcon_atomic_flush_pending() to perform flushing of all
registered nbcon consoles using their write_atomic() callback.
Unlike console_flush_all(), nbcon_atomic_flush_pending() will
only flush up through the newest record at the time of the
call. This prevents a CPU from printing unbounded when other
CPUs are adding records.
Also unlike console_flush_all(), nbcon_atomic_flush_pending()
will fully flush one console before flushing the next. This
helps to guarantee that a block of pending records (such as
a stack trace in an emergency situation) can be printed
atomically at once before releasing console ownership.
nbcon_atomic_flush_pending() is safe in any context because it
uses write_atomic() and acquires with unsafe_takeover disabled.
Use it in console_flush_on_panic() before flushing legacy
consoles. The legacy write() callbacks are not fully safe when
oops_in_progress is set.
Co-developed-by: John Ogness <john.ogness@...utronix.de>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@...utronix.de>
---
kernel/printk/internal.h | 2 +
kernel/printk/nbcon.c | 104 ++++++++++++++++++++++++++++++++++++++-
kernel/printk/printk.c | 2 +
3 files changed, 106 insertions(+), 2 deletions(-)
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index b7a0072eb2a4..a8df764fd0c5 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -84,6 +84,7 @@ void nbcon_seq_force(struct console *con, u64 seq);
bool nbcon_alloc(struct console *con);
void nbcon_init(struct console *con);
void nbcon_free(struct console *con);
+void nbcon_atomic_flush_pending(void);
/*
* Check if the given console is currently capable and allowed to print
@@ -138,6 +139,7 @@ static inline void nbcon_seq_force(struct console *con, u64 seq) { }
static inline bool nbcon_alloc(struct console *con) { return false; }
static inline void nbcon_init(struct console *con) { }
static inline void nbcon_free(struct console *con) { }
+static inline void nbcon_atomic_flush_pending(void) { }
static inline bool console_is_usable(struct console *con, short flags) { return false; }
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 1de6062d4ce3..fcdab2eaaedb 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -548,7 +548,6 @@ static struct printk_buffers panic_nbcon_pbufs;
* in an unsafe state. Otherwise, on success the caller may assume
* the console is not in an unsafe state.
*/
-__maybe_unused
static bool nbcon_context_try_acquire(struct nbcon_context *ctxt)
{
unsigned int cpu = smp_processor_id();
@@ -850,7 +849,6 @@ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
* When true is returned, @wctxt->ctxt.backlog indicates whether there are
* still records pending in the ringbuffer,
*/
-__maybe_unused
static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
{
struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
@@ -937,6 +935,108 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
return nbcon_context_exit_unsafe(ctxt);
}
+/**
+ * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
+ * write_atomic() callback
+ * @con: The nbcon console to flush
+ * @stop_seq: Flush up until this record
+ *
+ * Return: True if taken over while printing. Otherwise false.
+ *
+ * If flushing up to @stop_seq was not successful, it only makes sense for the
+ * caller to try again when true was returned. When false is returned, either
+ * there are no more records available to read or this context is not allowed
+ * to acquire the console.
+ */
+static bool __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+{
+ struct nbcon_write_context wctxt = { };
+ struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+
+ ctxt->console = con;
+ ctxt->spinwait_max_us = 2000;
+ ctxt->prio = NBCON_PRIO_NORMAL;
+
+ if (!nbcon_context_try_acquire(ctxt))
+ return false;
+
+ while (nbcon_seq_read(con) < stop_seq) {
+ /*
+ * nbcon_emit_next_record() returns false when the console was
+ * handed over or taken over. In both cases the context is no
+ * longer valid.
+ */
+ if (!nbcon_emit_next_record(&wctxt))
+ return true;
+
+ if (!ctxt->backlog)
+ break;
+ }
+
+ nbcon_context_release(ctxt);
+
+ return false;
+}
+
+/**
+ * __nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ * write_atomic() callback
+ * @stop_seq: Flush up until this record
+ */
+static void __nbcon_atomic_flush_pending(u64 stop_seq)
+{
+ struct console *con;
+ bool should_retry;
+ int cookie;
+
+ do {
+ should_retry = false;
+
+ cookie = console_srcu_read_lock();
+ for_each_console_srcu(con) {
+ short flags = console_srcu_read_flags(con);
+ unsigned long irq_flags;
+
+ if (!(flags & CON_NBCON))
+ continue;
+
+ if (!console_is_usable(con, flags))
+ continue;
+
+ if (nbcon_seq_read(con) >= stop_seq)
+ continue;
+
+ /*
+ * Atomic flushing does not use console driver
+ * synchronization (i.e. it does not hold the port
+ * lock for uart consoles). Therefore IRQs must be
+ * disabled to avoid being interrupted and then
+ * calling into a driver that will deadlock trying
+ * to acquire console ownership.
+ */
+ local_irq_save(irq_flags);
+
+ should_retry |= __nbcon_atomic_flush_pending_con(con, stop_seq);
+
+ local_irq_restore(irq_flags);
+ }
+ console_srcu_read_unlock(cookie);
+ } while (should_retry);
+}
+
+/**
+ * nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ * write_atomic() callback
+ *
+ * Flush the backlog up through the currently newest record. Any new
+ * records added while flushing will not be flushed. This is to avoid
+ * one CPU printing unbounded because other CPUs continue to add records.
+ */
+void nbcon_atomic_flush_pending(void)
+{
+ __nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+}
+
/**
* nbcon_alloc - Allocate buffers needed by the nbcon console
* @con: Console to allocate buffers for
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fe06856f7653..6404f2044ceb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3172,6 +3172,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
console_srcu_read_unlock(cookie);
}
+ nbcon_atomic_flush_pending();
+
console_flush_all(false, &next_seq, &handover);
}
--
2.39.2
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