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    Subject[PATCH 4.20 001/111] tty/ldsem: Wake up readers after timed out down_write()
    Date
    4.20-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Dmitry Safonov <dima@arista.com>

    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@wfg-t540p.sh.intel.com
    Link: lkml.kernel.org/r/20180907045041.GF1110@shao2-debian
    Cc: Jiri Slaby <jslaby@suse.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: stable@vger.kernel.org
    Reported-by: kernel test robot <rong.a.chen@intel.com>
    Signed-off-by: Dmitry Safonov <dima@arista.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/tty_ldsem.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    --- a/drivers/tty/tty_ldsem.c
    +++ b/drivers/tty/tty_ldsem.c
    @@ -293,6 +293,16 @@ down_write_failed(struct ld_semaphore *s
    if (!locked)
    atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
    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_current_state(TASK_RUNNING);

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