On 2021-07-02, Petr Mladek pmladek@suse.com wrote:
The standard printk() tries to flush the message to the console immediately. It tries to take the console lock. If the lock is already taken then the current owner is responsible for flushing even the new message.
There is a small race window between checking whether a new message is available and releasing the console lock. It is solved by re-checking the state after releasing the console lock. If the check is positive then console_unlock() tries to take the lock again and process the new message as well.
The commit 996e966640ddea7b535c ("printk: remove logbuf_lock") causes that console_seq is not longer read atomically. As a result, the re-check might be done with an inconsistent 64-bit index.
Solve it by using the last sequence number that has been checked under the console lock. In the worst case, it will take the lock again only to realized that the new message has already been proceed. But it was possible even before.
The variable next_seq is marked as __maybe_unused to call down compiler warning when CONFIG_PRINTK is not defined.
As Sergey already pointed out, this patch is not fixing a real problem. An inconsistent value (or an increased consistent value) would mean that another printer is actively printing, and thus a retry is not necessary anyway. But this patch will avoid a KASAN message about an unmarked (although safe) data race.
Fixes: commit 996e966640ddea7b535c ("printk: remove logbuf_lock") Reported-by: kernel test robot lkp@intel.com # unused next_seq warning Cc: stable@vger.kernel.org # 5.13 Signed-off-by: Petr Mladek pmladek@suse.com Acked-by: Sergey Senozhatsky senozhatsky@chromium.org
Reviewed-by: John Ogness john.ogness@linutronix.de