76 lines
3.3 KiB
Diff
76 lines
3.3 KiB
Diff
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From: John Ogness <john.ogness@linutronix.de>
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Date: Tue, 20 Aug 2024 08:35:38 +0206
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Subject: [PATCH 12/54] console: Improve console_srcu_read_flags() comments
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Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz
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It was not clear when exactly console_srcu_read_flags() must be
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used vs. directly reading @console->flags.
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Refactor and clarify that console_srcu_read_flags() is only
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needed if the console is registered or the caller is in a
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context where the registration status of the console may change
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(due to another context).
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The function requires the caller holds @console_srcu, which will
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ensure that the caller sees an appropriate @flags value for the
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registered console and that exit/cleanup routines will not run
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if the console is in the process of unregistration.
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Signed-off-by: John Ogness <john.ogness@linutronix.de>
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Reviewed-by: Petr Mladek <pmladek@suse.com>
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Link: https://lore.kernel.org/r/20240820063001.36405-13-john.ogness@linutronix.de
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Signed-off-by: Petr Mladek <pmladek@suse.com>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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include/linux/console.h | 28 +++++++++++++++++-----------
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1 file changed, 17 insertions(+), 11 deletions(-)
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--- a/include/linux/console.h
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+++ b/include/linux/console.h
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@@ -446,28 +446,34 @@ extern void console_list_unlock(void) __
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extern struct hlist_head console_list;
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/**
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- * console_srcu_read_flags - Locklessly read the console flags
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+ * console_srcu_read_flags - Locklessly read flags of a possibly registered
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+ * console
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* @con: struct console pointer of console to read flags from
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*
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- * This function provides the necessary READ_ONCE() and data_race()
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- * notation for locklessly reading the console flags. The READ_ONCE()
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- * in this function matches the WRITE_ONCE() when @flags are modified
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- * for registered consoles with console_srcu_write_flags().
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+ * Locklessly reading @con->flags provides a consistent read value because
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+ * there is at most one CPU modifying @con->flags and that CPU is using only
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+ * read-modify-write operations to do so.
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*
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- * Only use this function to read console flags when locklessly
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- * iterating the console list via srcu.
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+ * Requires console_srcu_read_lock to be held, which implies that @con might
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+ * be a registered console. The purpose of holding console_srcu_read_lock is
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+ * to guarantee that the console state is valid (CON_SUSPENDED/CON_ENABLED)
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+ * and that no exit/cleanup routines will run if the console is currently
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+ * undergoing unregistration.
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+ *
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+ * If the caller is holding the console_list_lock or it is _certain_ that
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+ * @con is not and will not become registered, the caller may read
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+ * @con->flags directly instead.
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*
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* Context: Any context.
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+ * Return: The current value of the @con->flags field.
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*/
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static inline short console_srcu_read_flags(const struct console *con)
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{
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WARN_ON_ONCE(!console_srcu_read_lock_is_held());
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/*
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- * Locklessly reading console->flags provides a consistent
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- * read value because there is at most one CPU modifying
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- * console->flags and that CPU is using only read-modify-write
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- * operations to do so.
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+ * The READ_ONCE() matches the WRITE_ONCE() when @flags are modified
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+ * for registered consoles with console_srcu_write_flags().
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*/
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return data_race(READ_ONCE(con->flags));
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}
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