[PATCH v2] tty: fix stall caused by missing memory barrier in drivers/tty/n_tty.c

From: Kosuke Tatsukawa
Date: Fri Oct 02 2015 - 04:27:47 EST

My colleague ran into a program stall on a x86_64 server, where
n_tty_read() was waiting for data even if there was data in the buffer
in the pty. kernel stack for the stuck process looks like below.
#0 [ffff88303d107b58] __schedule at ffffffff815c4b20
#1 [ffff88303d107bd0] schedule at ffffffff815c513e
#2 [ffff88303d107bf0] schedule_timeout at ffffffff815c7818
#3 [ffff88303d107ca0] wait_woken at ffffffff81096bd2
#4 [ffff88303d107ce0] n_tty_read at ffffffff8136fa23
#5 [ffff88303d107dd0] tty_read at ffffffff81368013
#6 [ffff88303d107e20] __vfs_read at ffffffff811a3704
#7 [ffff88303d107ec0] vfs_read at ffffffff811a3a57
#8 [ffff88303d107f00] sys_read at ffffffff811a4306
#9 [ffff88303d107f50] entry_SYSCALL_64_fastpath at ffffffff815c86d7

There seems to be two problems causing this issue.

First, in drivers/tty/n_tty.c, __receive_buf() stores the data and
updates ldata->commit_head using smp_store_release() and then checks
the wait queue using waitqueue_active(). However, since there is no
memory barrier, __receive_buf() could return without calling
wake_up_interactive_poll(), and at the same time, n_tty_read() could
start to wait in wait_woken() as in the following chart.

__receive_buf() n_tty_read()
------------------------------------------------------------------------
if (waitqueue_active(&tty->read_wait))
/* Memory operations issued after the
RELEASE may be completed before the
RELEASE operation has completed */
add_wait_queue(&tty->read_wait, &wait);
...
if (!input_available_p(tty, 0)) {
smp_store_release(&ldata->commit_head,
ldata->read_head);
...
timeout = wait_woken(&wait,
TASK_INTERRUPTIBLE, timeout);
------------------------------------------------------------------------

The second problem is that n_tty_read() also lacks a memory barrier
call and could also cause __receive_buf() to return without calling
wake_up_interactive_poll(), and n_tty_read() to wait in wait_woken()
as in the chart below.

__receive_buf() n_tty_read()
------------------------------------------------------------------------
spin_lock_irqsave(&q->lock, flags);
/* from add_wait_queue() */
...
if (!input_available_p(tty, 0)) {
/* Memory operations issued after the
RELEASE may be completed before the
RELEASE operation has completed */
smp_store_release(&ldata->commit_head,
ldata->read_head);
if (waitqueue_active(&tty->read_wait))
__add_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock,flags);
/* from add_wait_queue() */
...
timeout = wait_woken(&wait,
TASK_INTERRUPTIBLE, timeout);
------------------------------------------------------------------------

There are also other places in drivers/tty/n_tty.c which have similar
calls to waitqueue_active(), so instead of adding many memory barrier
calls, this patch simply removes the call to waitqueue_active(),
leaving just wake_up*() behind.

This fixes both problems because, even though the memory access before
or after the spinlocks in both wake_up*() and add_wait_queue() can
sneak into the critical section, it cannot go past it and the critical
section assures that they will be serialized (please see "INTER-CPU
ACQUIRING BARRIER EFFECTS" in Documentation/memory-barriers.txt for a
better explanation). Moreover, the resulting code is much simpler.

Latency measurement using a ping-pong test over a pty doesn't show any
visible performance drop.

Signed-off-by: Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
v2:
- Removed unnecessary hunks from v1 based on comments from Peter.
v1:
- https://lkml.org/lkml/2015/9/28/849
---
drivers/tty/n_tty.c | 15 +++++----------
1 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 20932cc..b09023b 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -343,8 +343,7 @@ static void n_tty_packet_mode_flush(struct tty_struct *tty)
spin_lock_irqsave(&tty->ctrl_lock, flags);
tty->ctrl_status |= TIOCPKT_FLUSHREAD;
spin_unlock_irqrestore(&tty->ctrl_lock, flags);
- if (waitqueue_active(&tty->link->read_wait))
- wake_up_interruptible(&tty->link->read_wait);
+ wake_up_interruptible(&tty->link->read_wait);
}
}

@@ -1382,8 +1381,7 @@ handle_newline:
put_tty_queue(c, ldata);
smp_store_release(&ldata->canon_head, ldata->read_head);
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
- if (waitqueue_active(&tty->read_wait))
- wake_up_interruptible_poll(&tty->read_wait, POLLIN);
+ wake_up_interruptible_poll(&tty->read_wait, POLLIN);
return 0;
}
}
@@ -1667,8 +1665,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,

if ((read_cnt(ldata) >= ldata->minimum_to_wake) || L_EXTPROC(tty)) {
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
- if (waitqueue_active(&tty->read_wait))
- wake_up_interruptible_poll(&tty->read_wait, POLLIN);
+ wake_up_interruptible_poll(&tty->read_wait, POLLIN);
}
}

@@ -1887,10 +1884,8 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
}

/* The termios change make the tty ready for I/O */
- if (waitqueue_active(&tty->write_wait))
- wake_up_interruptible(&tty->write_wait);
- if (waitqueue_active(&tty->read_wait))
- wake_up_interruptible(&tty->read_wait);
+ wake_up_interruptible(&tty->write_wait);
+ wake_up_interruptible(&tty->read_wait);
}

/**
--
1.7.1

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