[PATCH printk v5 00/40] reduce console_lock scope

[PATCH printk v5 00/40] reduce console_lock scope

[John Ogness]

This is v5 of a series to prepare for threaded/atomic
printing. v4 is here [0]. This series focuses on reducing the
scope of the BKL console_lock. It achieves this by switching to
SRCU and a dedicated mutex for console list iteration and
modification, respectively. The console_lock will no longer
offer this protection.

Also, during the review of v2 it came to our attention that
many console drivers are checking CON_ENABLED to see if they
are registered. Because this flag can change without
unregistering and because this flag does not represent an
atomic point when an (un)registration process is complete,
a new console_is_registered() function is introduced. This
function uses the console_list_lock to synchronize with the
(un)registration process to provide a reliable status.

All users of the console_lock for list iteration have been
modified. For the call sites where the console_lock is still
needed (for other reasons), comments are added to explain
exactly why the console_lock is needed.

All users of CON_ENABLED for registration status have been
modified to use console_is_registered(). Note that there are
still users of CON_ENABLED, but this is for legitimate purposes
about a registered console being able to print.

The base commit for this series is from Paul McKenney's RCU tree
and provides an NMI-safe SRCU implementation [1]. Without the
NMI-safe SRCU implementation, this series is not less safe than
mainline. But we will need the NMI-safe SRCU implementation for
atomic consoles anyway, so we might as well get it in
now. Especially since it _does_ increase the reliability for
mainline in the panic path.

Changes since v4:

printk:

- Introduce console_init_seq() to handle the now rather complex
  procedure to find an appropriate start sequence number for a
  new console upon registration.

- When registering a non-boot console and boot consoles are
  registered, try to flush all the consoles to get the next @seq
  value before falling back to use the @seq of the enabled boot
  console that is furthest behind.

- For console_force_preferred_locked(), make the console the
  head of the console list.

John Ogness

[0] https://lore.kernel.org/lkml/20221114162932.141883-1-john.ogness@linutronix.de
[1] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/log/?h=srcunmisafe.2022.11.09a

John Ogness (38):
  printk: Prepare for SRCU console list protection
  printk: register_console: use "registered" for variable names
  printk: move @seq initialization to helper
  printk: fix setting first seq for consoles
  um: kmsg_dump: only dump when no output console available
  tty: serial: kgdboc: document console_lock usage
  tty: tty_io: document console_lock usage
  proc: consoles: document console_lock usage
  printk: introduce console_list_lock
  console: introduce wrappers to read/write console flags
  um: kmsg_dumper: use srcu console list iterator
  kdb: use srcu console list iterator
  printk: console_flush_all: use srcu console list iterator
  printk: __pr_flush: use srcu console list iterator
  printk: console_is_usable: use console_srcu_read_flags
  printk: console_unblank: use srcu console list iterator
  printk: console_flush_on_panic: use srcu console list iterator
  printk: console_device: use srcu console list iterator
  console: introduce console_is_registered()
  serial_core: replace uart_console_enabled() with
    uart_console_registered()
  tty: nfcon: use console_is_registered()
  efi: earlycon: use console_is_registered()
  tty: hvc: use console_is_registered()
  tty: serial: earlycon: use console_is_registered()
  tty: serial: pic32_uart: use console_is_registered()
  tty: serial: samsung_tty: use console_is_registered()
  tty: serial: xilinx_uartps: use console_is_registered()
  usb: early: xhci-dbc: use console_is_registered()
  netconsole: avoid CON_ENABLED misuse to track registration
  printk, xen: fbfront: create/use safe function for forcing preferred
  tty: tty_io: use console_list_lock for list synchronization
  proc: consoles: use console_list_lock for list iteration
  tty: serial: kgdboc: use srcu console list iterator
  tty: serial: kgdboc: use console_list_lock for list traversal
  tty: serial: kgdboc: synchronize tty_find_polling_driver() and
    register_console()
  tty: serial: kgdboc: use console_list_lock to trap exit
  printk: relieve console_lock of list synchronization duties
  tty: serial: sh-sci: use setup() callback for early console

Thomas Gleixner (2):
  serial: kgdboc: Lock console list in probe function
  printk: Convert console_drivers list to hlist

 .clang-format                       |   1 +
 arch/m68k/emu/nfcon.c               |   9 +-
 arch/um/kernel/kmsg_dump.c          |  24 +-
 drivers/firmware/efi/earlycon.c     |   8 +-
 drivers/net/netconsole.c            |  21 +-
 drivers/tty/hvc/hvc_console.c       |   4 +-
 drivers/tty/serial/8250/8250_core.c |   2 +-
 drivers/tty/serial/earlycon.c       |   4 +-
 drivers/tty/serial/kgdboc.c         |  46 ++-
 drivers/tty/serial/pic32_uart.c     |   4 +-
 drivers/tty/serial/samsung_tty.c    |   2 +-
 drivers/tty/serial/serial_core.c    |  14 +-
 drivers/tty/serial/sh-sci.c         |  20 +-
 drivers/tty/serial/xilinx_uartps.c  |   2 +-
 drivers/tty/tty_io.c                |  18 +-
 drivers/usb/early/xhci-dbc.c        |   2 +-
 drivers/video/fbdev/xen-fbfront.c   |  12 +-
 fs/proc/consoles.c                  |  21 +-
 include/linux/console.h             | 129 +++++++-
 include/linux/serial_core.h         |  10 +-
 kernel/debug/kdb/kdb_io.c           |  18 +-
 kernel/printk/printk.c              | 493 +++++++++++++++++++++-------
 22 files changed, 680 insertions(+), 184 deletions(-)


base-commit: f733615e39aa2d6ddeef33b7b2c9aa6a5a2c2785
-- 
2.30.2


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[PATCH printk v5 07/40] um: kmsg_dump: only dump when no output console available

[John Ogness]

The initial intention of the UML kmsg_dumper is to dump the kernel
buffers to stdout if there is no console available to perform the
regular crash output.

However, if ttynull was registered as a console, no crash output was
seen. Commit e23fe90dec28 ("um: kmsg_dumper: always dump when not tty
console") tried to fix this by performing the kmsg_dump unless the
stdio console was behind /dev/console or enabled. But this allowed
kmsg dumping to occur even if other non-stdio consoles will output
the crash output. Also, a console being the driver behind
/dev/console has nothing to do with a crash scenario.

Restore the initial intention by dumping the kernel buffers to stdout
only if a non-ttynull console is registered and enabled. Also add
detailed comments so that it is clear why these rules are applied.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 arch/um/kernel/kmsg_dump.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
index 0224fcb36e22..40abf1e9ccb1 100644
--- a/arch/um/kernel/kmsg_dump.c
+++ b/arch/um/kernel/kmsg_dump.c
@@ -17,13 +17,22 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
 	unsigned long flags;
 	size_t len = 0;
 
-	/* only dump kmsg when no console is available */
+	/*
+	 * If no consoles are available to output crash information, dump
+	 * the kmsg buffer to stdout.
+	 */
+
 	if (!console_trylock())
 		return;
 
 	for_each_console(con) {
-		if(strcmp(con->name, "tty") == 0 &&
-		   (con->flags & (CON_ENABLED | CON_CONSDEV)) != 0) {
+		/*
+		 * The ttynull console and disabled consoles are ignored
+		 * since they cannot output. All other consoles are
+		 * expected to output the crash information.
+		 */
+		if (strcmp(con->name, "ttynull") != 0 &&
+		    (con->flags & CON_ENABLED)) {
 			break;
 		}
 	}
-- 
2.30.2


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[PATCH printk v5 13/40] um: kmsg_dumper: use srcu console list iterator

[John Ogness]

Rather than using the console_lock to guarantee safe console list
traversal, use srcu console list iteration.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 arch/um/kernel/kmsg_dump.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
index 40abf1e9ccb1..427dd5a61a38 100644
--- a/arch/um/kernel/kmsg_dump.c
+++ b/arch/um/kernel/kmsg_dump.c
@@ -16,29 +16,26 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
 	struct console *con;
 	unsigned long flags;
 	size_t len = 0;
+	int cookie;
 
 	/*
 	 * If no consoles are available to output crash information, dump
 	 * the kmsg buffer to stdout.
 	 */
 
-	if (!console_trylock())
-		return;
-
-	for_each_console(con) {
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(con) {
 		/*
 		 * The ttynull console and disabled consoles are ignored
 		 * since they cannot output. All other consoles are
 		 * expected to output the crash information.
 		 */
 		if (strcmp(con->name, "ttynull") != 0 &&
-		    (con->flags & CON_ENABLED)) {
+		    (console_srcu_read_flags(con) & CON_ENABLED)) {
 			break;
 		}
 	}
-
-	console_unlock();
-
+	console_srcu_read_unlock(cookie);
 	if (con)
 		return;
 
-- 
2.30.2


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Re: [PATCH printk v5 00/40] reduce console_lock scope

[Petr Mladek]

On Wed 2022-11-16 17:27:12, John Ogness wrote:
> This is v5 of a series to prepare for threaded/atomic
> printing. v4 is here [0]. This series focuses on reducing the
> scope of the BKL console_lock. It achieves this by switching to
> SRCU and a dedicated mutex for console list iteration and
> modification, respectively. The console_lock will no longer
> offer this protection.

The patchset looks ready for linux-next from my POV.

I am going to push it there right now to get as much testing
as possible before the merge window.

Any review and comments are still appreciate. We could always
take it back if some critical problems are discovered and
can't be solved easily.

Best Regards,
Petr

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Re: [PATCH printk v5 00/40] reduce console_lock scope

[Petr Mladek]

On Fri 2022-11-18 12:22:58, Petr Mladek wrote:
> On Wed 2022-11-16 17:27:12, John Ogness wrote:
> > This is v5 of a series to prepare for threaded/atomic
> > printing. v4 is here [0]. This series focuses on reducing the
> > scope of the BKL console_lock. It achieves this by switching to
> > SRCU and a dedicated mutex for console list iteration and
> > modification, respectively. The console_lock will no longer
> > offer this protection.
> 
> The patchset looks ready for linux-next from my POV.
> 
> I am going to push it there right now to get as much testing
> as possible before the merge window.

JFYI, the patchset is committed in printk/linux.git,
branch rework/console-list-lock.

I'll eventually merge it into rework/kthreads. But I wanted to have
it separated until it gets some more testing in linux-next and
eventually some more review.

Best Regards,
Petr

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Re: [PATCH printk v5 00/40] reduce console_lock scope

[Greg Kroah-Hartman]

On Wed, Nov 16, 2022 at 05:27:12PM +0106, John Ogness wrote:
> This is v5 of a series to prepare for threaded/atomic
> printing. v4 is here [0]. This series focuses on reducing the
> scope of the BKL console_lock. It achieves this by switching to
> SRCU and a dedicated mutex for console list iteration and
> modification, respectively. The console_lock will no longer
> offer this protection.
> 
> Also, during the review of v2 it came to our attention that
> many console drivers are checking CON_ENABLED to see if they
> are registered. Because this flag can change without
> unregistering and because this flag does not represent an
> atomic point when an (un)registration process is complete,
> a new console_is_registered() function is introduced. This
> function uses the console_list_lock to synchronize with the
> (un)registration process to provide a reliable status.
> 
> All users of the console_lock for list iteration have been
> modified. For the call sites where the console_lock is still
> needed (for other reasons), comments are added to explain
> exactly why the console_lock is needed.
> 
> All users of CON_ENABLED for registration status have been
> modified to use console_is_registered(). Note that there are
> still users of CON_ENABLED, but this is for legitimate purposes
> about a registered console being able to print.
> 
> The base commit for this series is from Paul McKenney's RCU tree
> and provides an NMI-safe SRCU implementation [1]. Without the
> NMI-safe SRCU implementation, this series is not less safe than
> mainline. But we will need the NMI-safe SRCU implementation for
> atomic consoles anyway, so we might as well get it in
> now. Especially since it _does_ increase the reliability for
> mainline in the panic path.
> 
> Changes since v4:
> 
> printk:
> 
> - Introduce console_init_seq() to handle the now rather complex
>   procedure to find an appropriate start sequence number for a
>   new console upon registration.
> 
> - When registering a non-boot console and boot consoles are
>   registered, try to flush all the consoles to get the next @seq
>   value before falling back to use the @seq of the enabled boot
>   console that is furthest behind.
> 
> - For console_force_preferred_locked(), make the console the
>   head of the console list.
> 


Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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