Re: [PATCHv5 2/7] tty/ldsem: Wake up readers after timed out down_write()

[Greg Kroah-Hartman]

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.ajdv4k6swzhvktl6@xxxxxxxxxxxxxxxxxxxxxx>
> Link: lkml.kernel.org/r/<20180907045041.GF1110@shao2-debian>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---

Why isn't this ok for the stable trees?

thanks,

greg k-h