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SubjectRe: [PATCH tty-next] n_tty: Fix termios_rwsem lockdep false positive
On (08/12/13 13:28), Artem Savkov wrote:
> 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(-)
> >

I hate to do this, but isn't it actually my patch posted here
https://lkml.org/lkml/2013/8/1/510

which was tagged as `wrong'?

-ss

> > 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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