From cf5543870186d6f99b631faaeca27beaa996d52f Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Wed, 4 Jun 2025 16:20:45 +0200 Subject: printk: Allow to use the printk kthread immediately even for 1st nbcon The kthreads for nbcon consoles are created by nbcon_alloc() at the beginning of the console registration. But it currently works only for the 2nd or later nbcon console because the code checks @printk_kthreads_running. The kthread for the 1st registered nbcon console is created at the very end of register_console() by printk_kthreads_check_locked(). As a result, the entire log is replayed synchronously when the "enabled" message gets printed. It might block the boot for a long time with a slow serial console. Prevent the synchronous flush by creating the kthread even for the 1st nbcon console when it is safe (kthreads ready and no boot consoles). Also inform printk() to use the kthread by setting @printk_kthreads_running. Note that the kthreads already must be running when it is safe and this is not the 1st nbcon console. Symmetrically, clear @printk_kthreads_running when the last nbcon console was unregistered by nbcon_free(). This requires updating @have_nbcon_console before nbcon_free() gets called. Note that there is _no_ problem when the 1st nbcon console replaces boot consoles. In this case, the kthread will be started at the end of registration after the boot consoles are removed. But the console does not reply the entire log buffer in this case. Note that the flag CON_PRINTBUFFER is always cleared when the boot consoles are removed and vice versa. Closes: https://lore.kernel.org/r/20250514173514.2117832-1-mcobb@thegoodpenguin.co.uk Tested-by: Michael Cobb Reviewed-by: John Ogness Link: https://patch.msgid.link/20250604142045.253301-1-pmladek@suse.com Signed-off-by: Petr Mladek --- kernel/printk/internal.h | 2 ++ kernel/printk/nbcon.c | 26 ++++++++++++++++++++++++-- kernel/printk/printk.c | 20 +++++++++++--------- 3 files changed, 37 insertions(+), 11 deletions(-) (limited to 'kernel') diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index 48a24e7b309d..567c9e100d47 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -64,6 +64,7 @@ struct dev_printk_info; extern struct printk_ringbuffer *prb; extern bool printk_kthreads_running; +extern bool printk_kthreads_ready; extern bool debug_non_panic_cpus; __printf(4, 0) @@ -180,6 +181,7 @@ static inline void nbcon_kthread_wake(struct console *con) #define PRINTKRB_RECORD_MAX 0 #define printk_kthreads_running (false) +#define printk_kthreads_ready (false) /* * In !PRINTK builds we still export console_sem diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index fd12efcc4aed..d60596777d27 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -1671,6 +1671,9 @@ bool nbcon_alloc(struct console *con) { struct nbcon_state state = { }; + /* Synchronize the kthread start. */ + lockdep_assert_console_list_lock_held(); + /* The write_thread() callback is mandatory. */ if (WARN_ON(!con->write_thread)) return false; @@ -1701,12 +1704,15 @@ bool nbcon_alloc(struct console *con) return false; } - if (printk_kthreads_running) { + if (printk_kthreads_ready && !have_boot_console) { if (!nbcon_kthread_create(con)) { kfree(con->pbufs); con->pbufs = NULL; return false; } + + /* Might be the first kthread. */ + printk_kthreads_running = true; } } @@ -1716,14 +1722,30 @@ bool nbcon_alloc(struct console *con) /** * nbcon_free - Free and cleanup the nbcon console specific data * @con: Console to free/cleanup nbcon data + * + * Important: @have_nbcon_console must be updated before calling + * this function. In particular, it can be set only when there + * is still another nbcon console registered. */ void nbcon_free(struct console *con) { struct nbcon_state state = { }; - if (printk_kthreads_running) + /* Synchronize the kthread stop. */ + lockdep_assert_console_list_lock_held(); + + if (printk_kthreads_running) { nbcon_kthread_stop(con); + /* Might be the last nbcon console. + * + * Do not rely on printk_kthreads_check_locked(). It is not + * called in some code paths, see nbcon_free() callers. + */ + if (!have_nbcon_console) + printk_kthreads_running = false; + } + nbcon_state_set(con, &state); /* Boot consoles share global printk buffers. */ diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 1eea80d0648e..0efbcdda9aab 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3574,7 +3574,7 @@ EXPORT_SYMBOL(console_resume); static int unregister_console_locked(struct console *console); /* True when system boot is far enough to create printer threads. */ -static bool printk_kthreads_ready __ro_after_init; +bool printk_kthreads_ready __ro_after_init; static struct task_struct *printk_legacy_kthread; @@ -3713,6 +3713,7 @@ static void printk_kthreads_check_locked(void) if (!printk_kthreads_ready) return; + /* Start or stop the legacy kthread when needed. */ if (have_legacy_console || have_boot_console) { if (!printk_legacy_kthread && force_legacy_kthread() && @@ -4204,14 +4205,6 @@ static int unregister_console_locked(struct console *console) */ synchronize_srcu(&console_srcu); - if (console->flags & CON_NBCON) - nbcon_free(console); - - console_sysfs_notify(); - - if (console->exit) - res = console->exit(console); - /* * With this console gone, the global flags tracking registered * console types may have changed. Update them. @@ -4232,6 +4225,15 @@ static int unregister_console_locked(struct console *console) if (!found_nbcon_con) have_nbcon_console = found_nbcon_con; + /* @have_nbcon_console must be updated before calling nbcon_free(). */ + if (console->flags & CON_NBCON) + nbcon_free(console); + + console_sysfs_notify(); + + if (console->exit) + res = console->exit(console); + /* Changed console list, may require printer threads to start/stop. */ printk_kthreads_check_locked(); -- cgit v1.2.3 From 571c1ea91a73db56bd94054fabecd0f070dc90db Mon Sep 17 00:00:00 2001 From: John Ogness Date: Fri, 6 Jun 2025 21:01:49 +0206 Subject: printk: nbcon: Allow reacquire during panic If a console printer is interrupted during panic, it will never be able to reacquire ownership in order to perform and cleanup. That in itself is not a problem, since the non-panic CPU will simply quiesce in an endless loop within nbcon_reacquire_nobuf(). However, in this state, platforms that do not support a true NMI to interrupt the quiesced CPU will not be able to shutdown that CPU from within panic(). This then causes problems for such as being unable to load and run a kdump kernel. Fix this by allowing non-panic CPUs to reacquire ownership using a direct acquire. Then the non-panic CPUs can successfullyl exit the nbcon_reacquire_nobuf() loop and the console driver can perform any necessary cleanup. But more importantly, the CPU is no longer quiesced and is free to process any interrupts necessary for panic() to shutdown the CPU. All other forms of acquire are still not allowed for non-panic CPUs since it is safer to have them avoid gaining console ownership that is not strictly necessary. Reported-by: Michael Kelley Closes: https://lore.kernel.org/r/SN6PR02MB4157A4C5E8CB219A75263A17D46DA@SN6PR02MB4157.namprd02.prod.outlook.com Signed-off-by: John Ogness Reviewed-by: Petr Mladek Tested-by: Michael Kelley Link: https://patch.msgid.link/20250606185549.900611-1-john.ogness@linutronix.de Signed-off-by: Petr Mladek --- kernel/printk/nbcon.c | 63 +++++++++++++++++++++++++++++++++------------------ 1 file changed, 41 insertions(+), 22 deletions(-) (limited to 'kernel') diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index fd12efcc4aed..e7a3af81b173 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -214,8 +214,9 @@ static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq) /** * nbcon_context_try_acquire_direct - Try to acquire directly - * @ctxt: The context of the caller - * @cur: The current console state + * @ctxt: The context of the caller + * @cur: The current console state + * @is_reacquire: This acquire is a reacquire * * Acquire the console when it is released. Also acquire the console when * the current owner has a lower priority and the console is in a safe state. @@ -225,17 +226,17 @@ static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq) * * Errors: * - * -EPERM: A panic is in progress and this is not the panic CPU. - * Or the current owner or waiter has the same or higher - * priority. No acquire method can be successful in - * this case. + * -EPERM: A panic is in progress and this is neither the panic + * CPU nor is this a reacquire. Or the current owner or + * waiter has the same or higher priority. No acquire + * method can be successful in these cases. * * -EBUSY: The current owner has a lower priority but the console * in an unsafe state. The caller should try using * the handover acquire method. */ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt, - struct nbcon_state *cur) + struct nbcon_state *cur, bool is_reacquire) { unsigned int cpu = smp_processor_id(); struct console *con = ctxt->console; @@ -243,14 +244,20 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt, do { /* - * Panic does not imply that the console is owned. However, it - * is critical that non-panic CPUs during panic are unable to - * acquire ownership in order to satisfy the assumptions of - * nbcon_waiter_matches(). In particular, the assumption that - * lower priorities are ignored during panic. + * Panic does not imply that the console is owned. However, + * since all non-panic CPUs are stopped during panic(), it + * is safer to have them avoid gaining console ownership. + * + * If this acquire is a reacquire (and an unsafe takeover + * has not previously occurred) then it is allowed to attempt + * a direct acquire in panic. This gives console drivers an + * opportunity to perform any necessary cleanup if they were + * interrupted by the panic CPU while printing. */ - if (other_cpu_in_panic()) + if (other_cpu_in_panic() && + (!is_reacquire || cur->unsafe_takeover)) { return -EPERM; + } if (ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio) return -EPERM; @@ -301,8 +308,9 @@ static bool nbcon_waiter_matches(struct nbcon_state *cur, int expected_prio) * Event #1 implies this context is EMERGENCY. * Event #2 implies the new context is PANIC. * Event #3 occurs when panic() has flushed the console. - * Events #4 and #5 are not possible due to the other_cpu_in_panic() - * check in nbcon_context_try_acquire_direct(). + * Event #4 occurs when a non-panic CPU reacquires. + * Event #5 is not possible due to the other_cpu_in_panic() check + * in nbcon_context_try_acquire_handover(). */ return (cur->req_prio == expected_prio); @@ -431,6 +439,16 @@ static int nbcon_context_try_acquire_handover(struct nbcon_context *ctxt, WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio); WARN_ON_ONCE(!cur->unsafe); + /* + * Panic does not imply that the console is owned. However, it + * is critical that non-panic CPUs during panic are unable to + * wait for a handover in order to satisfy the assumptions of + * nbcon_waiter_matches(). In particular, the assumption that + * lower priorities are ignored during panic. + */ + if (other_cpu_in_panic()) + return -EPERM; + /* Handover is not possible on the same CPU. */ if (cur->cpu == cpu) return -EBUSY; @@ -558,7 +576,8 @@ static struct printk_buffers panic_nbcon_pbufs; /** * nbcon_context_try_acquire - Try to acquire nbcon console - * @ctxt: The context of the caller + * @ctxt: The context of the caller + * @is_reacquire: This acquire is a reacquire * * Context: Under @ctxt->con->device_lock() or local_irq_save(). * Return: True if the console was acquired. False otherwise. @@ -568,7 +587,7 @@ 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. */ -static bool nbcon_context_try_acquire(struct nbcon_context *ctxt) +static bool nbcon_context_try_acquire(struct nbcon_context *ctxt, bool is_reacquire) { unsigned int cpu = smp_processor_id(); struct console *con = ctxt->console; @@ -577,7 +596,7 @@ static bool nbcon_context_try_acquire(struct nbcon_context *ctxt) nbcon_state_read(con, &cur); try_again: - err = nbcon_context_try_acquire_direct(ctxt, &cur); + err = nbcon_context_try_acquire_direct(ctxt, &cur, is_reacquire); if (err != -EBUSY) goto out; @@ -913,7 +932,7 @@ void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt); - while (!nbcon_context_try_acquire(ctxt)) + while (!nbcon_context_try_acquire(ctxt, true)) cpu_relax(); nbcon_write_context_set_buf(wctxt, NULL, 0); @@ -1101,7 +1120,7 @@ static bool nbcon_emit_one(struct nbcon_write_context *wctxt, bool use_atomic) cant_migrate(); } - if (!nbcon_context_try_acquire(ctxt)) + if (!nbcon_context_try_acquire(ctxt, false)) goto out; /* @@ -1486,7 +1505,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq, ctxt->prio = nbcon_get_default_prio(); ctxt->allow_unsafe_takeover = allow_unsafe_takeover; - if (!nbcon_context_try_acquire(ctxt)) + if (!nbcon_context_try_acquire(ctxt, false)) return -EPERM; while (nbcon_seq_read(con) < stop_seq) { @@ -1762,7 +1781,7 @@ bool nbcon_device_try_acquire(struct console *con) ctxt->console = con; ctxt->prio = NBCON_PRIO_NORMAL; - if (!nbcon_context_try_acquire(ctxt)) + if (!nbcon_context_try_acquire(ctxt, false)) return false; if (!nbcon_context_enter_unsafe(ctxt)) -- cgit v1.2.3