mirrors/linux
1
0
Fork 0
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git synced 2026-05-05 17:03:47 -04:00
Code Issues Packages Projects Releases Wiki Activity

console: Improve console_srcu_read_flags() comments

Browse Source 
It was not clear when exactly console_srcu_read_flags() must be
used vs. directly reading @console->flags.

Refactor and clarify that console_srcu_read_flags() is only
needed if the console is registered or the caller is in a
context where the registration status of the console may change
(due to another context).

The function requires the caller holds @console_srcu, which will
ensure that the caller sees an appropriate @flags value for the
registered console and that exit/cleanup routines will not run
if the console is in the process of unregistration.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-13-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
John Ogness
2024-08-20 08:35:38 +02:06
committed by Petr Mladek
parent eabd4600da
commit dc219d8d85
1 changed files with 17 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

28
include/linux/console.h
View File

@@ -446,28 +446,34 @@ extern void console_list_unlock(void) __releases(console_mutex);
extern struct hlist_head console_list;
/**
* console_srcu_read_flags - Locklessly read the console flags
* console_srcu_read_flags - Locklessly read flags of a possibly registered
* console
* @con: struct console pointer of console to read flags from
*
* This function provides the necessary READ_ONCE() and data_race()
* notation for locklessly reading the console flags. The READ_ONCE()
* in this function matches the WRITE_ONCE() when @flags are modified
* for registered consoles with console_srcu_write_flags().
* Locklessly reading @con->flags provides a consistent read value because
* there is at most one CPU modifying @con->flags and that CPU is using only
* read-modify-write operations to do so.
*
* Only use this function to read console flags when locklessly
* iterating the console list via srcu.
* Requires console_srcu_read_lock to be held, which implies that @con might
* be a registered console. The purpose of holding console_srcu_read_lock is
* to guarantee that the console state is valid (CON_SUSPENDED/CON_ENABLED)
* and that no exit/cleanup routines will run if the console is currently
* undergoing unregistration.
*
* If the caller is holding the console_list_lock or it is _certain_ that
* @con is not and will not become registered, the caller may read
* @con->flags directly instead.
*
* Context: Any context.
* Return: The current value of the @con->flags field.
*/
static inline short console_srcu_read_flags(const struct console *con)
{
WARN_ON_ONCE(!console_srcu_read_lock_is_held());
/*
* Locklessly reading console->flags provides a consistent
* read value because there is at most one CPU modifying
* console->flags and that CPU is using only read-modify-write
* operations to do so.
* The READ_ONCE() matches the WRITE_ONCE() when @flags are modified
* for registered consoles with console_srcu_write_flags().
*/
return data_race(READ_ONCE(con->flags));
}