Patch "tty: tty_buffer: Fix the softlockup issue in flush_to_ldisc" has been added to the 5.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    tty: tty_buffer: Fix the softlockup issue in flush_to_ldisc

to the 5.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tty-tty_buffer-fix-the-softlockup-issue-in-flush_to_.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 51f045ca120e816ca85bbedc3ad992105aa6d50c
Author: Guanghui Feng <guanghuifeng@xxxxxxxxxxxxxxxxx>
Date:   Mon Oct 11 22:08:24 2021 +0800

    tty: tty_buffer: Fix the softlockup issue in flush_to_ldisc
    
    [ Upstream commit 3968ddcf05fb4b9409cd1859feb06a5b0550a1c1 ]
    
    When running ltp testcase(ltp/testcases/kernel/pty/pty04.c) with arm64, there is a soft lockup,
    which look like this one:
    
      Workqueue: events_unbound flush_to_ldisc
      Call trace:
       dump_backtrace+0x0/0x1ec
       show_stack+0x24/0x30
       dump_stack+0xd0/0x128
       panic+0x15c/0x374
       watchdog_timer_fn+0x2b8/0x304
       __run_hrtimer+0x88/0x2c0
       __hrtimer_run_queues+0xa4/0x120
       hrtimer_interrupt+0xfc/0x270
       arch_timer_handler_phys+0x40/0x50
       handle_percpu_devid_irq+0x94/0x220
       __handle_domain_irq+0x88/0xf0
       gic_handle_irq+0x84/0xfc
       el1_irq+0xc8/0x180
       slip_unesc+0x80/0x214 [slip]
       tty_ldisc_receive_buf+0x64/0x80
       tty_port_default_receive_buf+0x50/0x90
       flush_to_ldisc+0xbc/0x110
       process_one_work+0x1d4/0x4b0
       worker_thread+0x180/0x430
       kthread+0x11c/0x120
    
    In the testcase pty04, The first process call the write syscall to send
    data to the pty master. At the same time, the workqueue will do the
    flush_to_ldisc to pop data in a loop until there is no more data left.
    When the sender and workqueue running in different core, the sender sends
    data fastly in full time which will result in workqueue doing work in loop
    for a long time and occuring softlockup in flush_to_ldisc with kernel
    configured without preempt. So I add need_resched check and cond_resched
    in the flush_to_ldisc loop to avoid it.
    
    Signed-off-by: Guanghui Feng <guanghuifeng@xxxxxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/1633961304-24759-1-git-send-email-guanghuifeng@xxxxxxxxxxxxxxxxx
    Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index ec145a59f1993..bb148dbfbb88f 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -534,6 +534,9 @@ static void flush_to_ldisc(struct work_struct *work)
 		if (!count)
 			break;
 		head->read += count;
+
+		if (need_resched())
+			cond_resched();
 	}
 
 	mutex_unlock(&buf->lock);