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SubjectRe: [PATCH tty-next] n_tty: Fix termios_rwsem lockdep false positive
Hi Peter,

On Sun, Aug 11, 2013 at 08:04:23AM -0400, Peter Hurley wrote:
> Lockdep reports a circular lock dependency between
> atomic_read_lock and termios_rwsem [1]. However, a lock
> order deadlock is not possible since CPU1 only holds a
> read lock which cannot prevent CPU0 from also acquiring
> a read lock on the same r/w semaphore.
>
> Unfortunately, lockdep cannot currently distinguish whether
> the locks are read or write for any particular lock graph,
> merely that the locks _were_ previously read and/or write.
>
> Until lockdep is fixed, re-order atomic_read_lock so
> termios_rwsem can be dropped and reacquired without
> triggering lockdep.

Works fine, thanks.

Reported-and-tested-by: Artem Savkov <artem.savkov@gmail.com>

> Reported-by: Artem Savkov <artem.savkov@gmail.com>
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
>
> [1] Initial lockdep report from Artem Savkov <artem.savkov@gmail.com>
>
> ======================================================
> [ INFO: possible circular locking dependency detected ]
> 3.11.0-rc3-next-20130730+ #140 Tainted: G W
> -------------------------------------------------------
> bash/1198 is trying to acquire lock:
> (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
>
> but task is already holding lock:
> (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (&ldata->atomic_read_lock){+.+...}:
> [<ffffffff811111cc>] validate_chain+0x73c/0x850
> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540
> [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> [<ffffffff811f2702>] SyS_read+0x62/0xa0
> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
>
> -> #0 (&tty->termios_rwsem){++++..}:
> [<ffffffff8111064f>] check_prev_add+0x14f/0x590
> [<ffffffff811111cc>] validate_chain+0x73c/0x850
> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> [<ffffffff81d372c1>] down_read+0x51/0xa0
> [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> [<ffffffff811f2702>] SyS_read+0x62/0xa0
> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&ldata->atomic_read_lock);
> lock(&tty->termios_rwsem);
> lock(&ldata->atomic_read_lock);
> lock(&tty->termios_rwsem);
>
> *** DEADLOCK ***
>
> 2 locks held by bash/1198:
> #0: (&tty->ldisc_sem){.+.+.+}, at: [<ffffffff816ade04>] tty_ldisc_ref_wait+0x24/0x60
> #1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
>
> stack backtrace:
> CPU: 1 PID: 1198 Comm: bash Tainted: G W 3.11.0-rc3-next-20130730+ #140
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
> 0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002
> 0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98
> ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670
> Call Trace:
> [<ffffffff81d34074>] dump_stack+0x59/0x7d
> [<ffffffff8110ed75>] print_circular_bug+0x105/0x120
> [<ffffffff8111064f>] check_prev_add+0x14f/0x590
> [<ffffffff81d3ab5f>] ? _raw_spin_unlock_irq+0x4f/0x70
> [<ffffffff811111cc>] validate_chain+0x73c/0x850
> [<ffffffff8110ae0f>] ? trace_hardirqs_off_caller+0x1f/0x190
> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
> [<ffffffff81d372c1>] down_read+0x51/0xa0
> [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
> [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
> [<ffffffff810e4130>] ? try_to_wake_up+0x210/0x210
> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> [<ffffffff811f2702>] SyS_read+0x62/0xa0
> [<ffffffff815e24ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
> ---
> drivers/tty/n_tty.c | 25 +++++++++++--------------
> 1 file changed, 11 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index dd8ae0c..c9a9ddd 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -2122,6 +2122,17 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
> if (c < 0)
> return c;
>
> + /*
> + * Internal serialization of reads.
> + */
> + if (file->f_flags & O_NONBLOCK) {
> + if (!mutex_trylock(&ldata->atomic_read_lock))
> + return -EAGAIN;
> + } else {
> + if (mutex_lock_interruptible(&ldata->atomic_read_lock))
> + return -ERESTARTSYS;
> + }
> +
> down_read(&tty->termios_rwsem);
>
> minimum = time = 0;
> @@ -2141,20 +2152,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
> }
> }
>
> - /*
> - * Internal serialization of reads.
> - */
> - if (file->f_flags & O_NONBLOCK) {
> - if (!mutex_trylock(&ldata->atomic_read_lock)) {
> - up_read(&tty->termios_rwsem);
> - return -EAGAIN;
> - }
> - } else {
> - if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
> - up_read(&tty->termios_rwsem);
> - return -ERESTARTSYS;
> - }
> - }
> packet = tty->packet;
>
> add_wait_queue(&tty->read_wait, &wait);
> --
> 1.8.1.2
>

--
Regards,
Artem


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