4.4-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 @@ -307,6 +307,16 @@ 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);