On Tue, Sep 18, 2018 at 12:52:53AM +0100, Dmitry Safonov wrote:
> ldsem_down_read() will sleep if there is pending writer in the queue.
> If the writer times out, readers in the queue should be woken up,
> otherwise they may miss a chance to acquire the semaphore until the last
> active reader will do ldsem_up_read().
>
> There was a couple of reports where there was one active reader and
> other readers soft locked up:
> Showing all locks held in the system:
> 2 locks held by khungtaskd/17:
> #0: (rcu_read_lock){......}, at: watchdog+0x124/0x6d1
> #1: (tasklist_lock){.+.+..}, at: debug_show_all_locks+0x72/0x2d3
> 2 locks held by askfirst/123:
> #0: (&tty->ldisc_sem){.+.+.+}, at: ldsem_down_read+0x46/0x58
> #1: (&ldata->atomic_read_lock){+.+...}, at: n_tty_read+0x115/0xbe4
>
> Prevent readers wait for active readers to release ldisc semaphore.
>
> Link:
> lkml.kernel.org/r/<20171121132855.ajdv4k6swzhvk...@wfg-t540p.sh.intel.com>
> Link: lkml.kernel.org/r/<20180907045041.GF1110@shao2-debian>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Jiri Slaby <jsl...@suse.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Reported-by: kernel test robot <rong.a.c...@intel.com>
> Signed-off-by: Dmitry Safonov <d...@arista.com>
> ---
Why isn't this ok for the stable trees?
thanks,
greg k-h
