Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH v2] printk/console: Check consistent sequence number when handling race in console_unlock() | Date | Sat, 03 Jul 2021 08:32:02 +0206 |
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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>
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