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authorDmitry Safonov <dima@arista.com>2018-11-01 03:24:46 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2018-12-05 14:16:33 +0300
commit231f8fd0cca078bd4396dd7e380db813ac5736e2 (patch)
treef4165fe647815ea4b48241ae34aa328b44f93f32
parent77cab92a2cb15bcbdd7be0af773799e92d6a8546 (diff)
downloadlinux-231f8fd0cca078bd4396dd7e380db813ac5736e2.tar.xz
tty/ldsem: Wake up readers after timed out down_write()
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>
-rw-r--r--drivers/tty/tty_ldsem.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 0c98d88f795a..b989ca26fc78 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -293,6 +293,16 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
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);