Re: [PATCH 3.16 23/99] tty/ldsem: Wake up readers after timed out down_write()

[Dmitry Safonov]

Hi Ben,

JFI: I haven't checked your tree, but the same patches set contains fixes
that are more important to my mind (looking at 4.9 stable tree):
- "tty: Drop tty->count on tty_reopen() failure" commit fe3241679009
- "tty: Hold tty_ldisc_lock() during tty_reopen()" commit 83d817f41070
  with follow-up fixup "tty: Don't hold ldisc lock in tty_reopen() if
ldisc present"
  commit d3736d82e816

So, I don't know your policy about choosing backports, but I thought worth
to mention those patches as you might like to glance at them.
(maybe they are already in your tree)

Thanks,
Dmitry

On Tue, 2 Apr 2019 at 14:44, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
>
> 3.16.65-rc1 review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Dmitry Safonov <dima@xxxxxxxxxx>
>
> commit 231f8fd0cca078bd4396dd7e380db813ac5736e2 upstream.
>
> 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: 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>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
>  drivers/tty/tty_ldsem.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> --- a/drivers/tty/tty_ldsem.c
> +++ b/drivers/tty/tty_ldsem.c
> @@ -306,6 +306,16 @@ down_write_failed(struct ld_semaphore *s
>         if (!locked)
>                 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
>         list_del(&waiter.list);
> +
> +       /*
> +        * In case of timeout, wake up every reader who gave the right of way
> +        * to writer. Prevent separation readers into two groups:
> +        * one that helds semaphore and another that sleeps.
> +        * (in case of no contention with a writer)
> +        */
> +       if (!locked && list_empty(&sem->write_wait))
> +               __ldsem_wake_readers(sem);
> +
>         raw_spin_unlock_irq(&sem->wait_lock);
>
>         __set_task_state(tsk, TASK_RUNNING);
>