Messages in this thread | | | Date | Thu, 8 Feb 2018 23:53:07 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH v2] printk: Relocate wake_klogd check close to the end of console_unlock() |
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On (02/08/18 14:04), Petr Mladek wrote: > We mark for waking up klogd whenever we see a new message sequence in > the main loop. However, the actual wakeup is always at the end of the > function and we can easily test for the wakeup condition when we do > the final should-we-repeat check. > > Move the wake_klogd condition check out of the main loop. This avoids > doing the same thing repeatedly and groups similar checks into a > common place. > > This fixes a race introduced by the commit dbdda842fe96f8932 ("printk: Add > console owner and waiter logic to load balance console writes"). > The current console owner might process the newly added message before > the related printk() start waiting for the console lock. Then the lock > is passed without waking klogd. The new owner sees the already updated > seen_seq and does not know that the wakeup is needed.
I need to do more "research" on this. I though about it some time ago, and I think that waking up klogd _only_ when we don't have any pending logbuf messages still can be pretty late. Can't it? We can spin in console_unlock() printing loop for a long time, probably passing console_sem ownership between CPUs, without waking up the log_wait waiter. May be we can wake it up from the printing loop, outside of logbuf_lock, and let klogd to compete for logbuf_lock with the printing CPU. Why do we wake it up only when we are done pushing messages to a potentially slow serial console?
-ss
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