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console: introduce wrappers to read/write console flags

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After switching to SRCU for console list iteration, some readers
will begin readings console->flags as a data race. Locklessly
reading console->flags provides a consistent value because there
is at most one CPU modifying console->flags and that CPU is
using only read-modify-write operations.

Introduce a wrapper for SRCU iterators to read console flags.
Introduce a matching wrapper to write to flags of registered
consoles. Writing to flags of registered consoles is synchronized
by the console_list_lock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-13-john.ogness@linutronix.de
This commit is contained in:
John Ogness
2022-11-16 17:27:24 +01:06
committed by Petr Mladek
parent 4dc64682ad
commit 100bdef2c1
2 changed files with 50 additions and 5 deletions
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10
kernel/printk/printk.c
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@@ -3069,7 +3069,7 @@ void console_stop(struct console *console)
__pr_flush(console, 1000, true);
console_list_lock();
console_lock();
console->flags &= ~CON_ENABLED;
console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
console_unlock();
console_list_unlock();
@@ -3087,7 +3087,7 @@ void console_start(struct console *console)
{
console_list_lock();
console_lock();
console->flags |= CON_ENABLED;
console_srcu_write_flags(console, console->flags | CON_ENABLED);
console_unlock();
console_list_unlock();
__pr_flush(console, 1000, true);
@@ -3343,7 +3343,7 @@ void register_console(struct console *newcon)
} else if (newcon->flags & CON_CONSDEV) {
/* Only the new head can have CON_CONSDEV set. */
console_first()->flags &= ~CON_CONSDEV;
console_srcu_write_flags(console_first(), console_first()->flags & ~CON_CONSDEV);
hlist_add_head_rcu(&newcon->node, &console_list);
} else {
@@ -3400,7 +3400,7 @@ static int unregister_console_locked(struct console *console)
console_lock();
/* Disable it unconditionally */
console->flags &= ~CON_ENABLED;
console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
if (hlist_unhashed(&console->node)) {
console_unlock();
@@ -3419,7 +3419,7 @@ static int unregister_console_locked(struct console *console)
* console has any device attached. Oh well....
*/
if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
console_first()->flags |= CON_CONSDEV;
console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV);
console_unlock();