lkml.org 
[lkml]   [2013]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    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
    >


    \
     
     \ /
      Last update: 2013-08-12 13:01    [W:2.296 / U:0.508 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site