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    SubjectRe: [PATCH] n_tty: release atomic_read_lock before calling schedule_timeout()
    On Tue, Jul 30, 2013 at 12:39:54PM -0400, Peter Hurley wrote:
    > On 07/30/2013 11:35 AM, Artem Savkov wrote:
    > >ldata->atomic_read_lock should be released before scheduling as well as
    > >tty->termios_rwsem, otherwise there is a potential deadlock detected by lockdep
    >
    > False positive.
    >
    > >Introduced in "n_tty: Access termios values safely"
    > >(9356b535fcb71db494fc434acceb79f56d15bda2 in linux-next.git)
    > >
    > >[ 16.822058] ======================================================
    > >[ 16.822058] [ INFO: possible circular locking dependency detected ]
    > >[ 16.822058] 3.11.0-rc3-next-20130730+ #140 Tainted: G W
    > >[ 16.822058] -------------------------------------------------------
    > >[ 16.822058] bash/1198 is trying to acquire lock:
    > >[ 16.822058] (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
    > >[ 16.822058]
    > >[ 16.822058] but task is already holding lock:
    > >[ 16.822058] (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
    > >[ 16.822058]
    > >[ 16.822058] which lock already depends on the new lock.
    > >[ 16.822058]
    > >[ 16.822058]
    > >[ 16.822058] the existing dependency chain (in reverse order) is:
    > >[ 16.822058]
    > >-> #1 (&ldata->atomic_read_lock){+.+...}:
    > >[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
    > >[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
    > >[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
    > >[ 16.822058] [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540
    > >[ 16.822058] [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
    > >[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
    > >[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
    > >[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
    > >[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
    > >[ 16.822058]
    > >-> #0 (&tty->termios_rwsem){++++..}:
    > >[ 16.822058] [<ffffffff8111064f>] check_prev_add+0x14f/0x590
    > >[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
    > >[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
    > >[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
    > >[ 16.822058] [<ffffffff81d372c1>] down_read+0x51/0xa0
    > >[ 16.822058] [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
    > >[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
    > >[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
    > >[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
    > >[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
    > >[ 16.822058]
    > >[ 16.822058] other info that might help us debug this:
    > >[ 16.822058]
    > >[ 16.822058] Possible unsafe locking scenario:
    > >[ 16.822058]
    > >[ 16.822058] CPU0 CPU1
    > >[ 16.822058] ---- ----
    > >[ 16.822058] lock(&ldata->atomic_read_lock);
    > >[ 16.822058] lock(&tty->termios_rwsem);
    > >[ 16.822058] lock(&ldata->atomic_read_lock);
    > >[ 16.822058] lock(&tty->termios_rwsem);
    > >[ 16.822058]
    > >[ 16.822058] *** DEADLOCK ***
    >
    > This situation is not possible since termios_rwsem is a read/write semaphore;
    > CPU1 cannot prevent CPU0 from obtaining a read lock on termios_rwsem.
    Oops, yes, sorry.

    > This looks like a regression caused by:
    >
    > commit a51805efae5dda0da66f79268ffcf0715f9dbea4
    > Author: Michel Lespinasse <walken@google.com>
    > Date: Mon Jul 8 14:23:49 2013 -0700
    >
    > lockdep: Introduce lock_acquire_exclusive()/shared() helper macros
    Doesn't seem to be this commit. I see nothing wrong here and just to be
    sure I've checked the kernel with this commit reverted. The issue is
    still there.


    > In lockdep.h, the spinlock/mutex/rwsem/rwlock/lock_map acquire macros have
    > different definitions based on the value of CONFIG_PROVE_LOCKING. We have
    > separate ifdefs for each of these definitions, which seems redundant.
    >
    > Introduce lock_acquire_{exclusive,shared,shared_recursive} helpers which
    > will have different definitions based on CONFIG_PROVE_LOCKING. Then all
    > other helper macros can be defined based on the above ones, which reduces
    > the amount of ifdefined code.
    >
    > Signed-off-by: Michel Lespinasse <walken@google.com>
    > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    > Cc: Oleg Nesterov <oleg@redhat.com>
    > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
    > Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
    > Cc: Rusty Russell <rusty@rustcorp.com.au>
    > Cc: Andi Kleen <ak@linux.intel.com>
    > Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Link: http://lkml.kernel.org/r/20130708212350.6DD1931C15E@corp2gmr1-1.hot.corp.google.com
    > Signed-off-by: Ingo Molnar <mingo@kernel.org>
    >
    >
    > >[ 16.822058]
    > >[ 16.822058] 2 locks held by bash/1198:
    > >[ 16.822058] #0: (&tty->ldisc_sem){.+.+.+}, at: [<ffffffff816ade04>] tty_ldisc_ref_wait+0x24/0x60
    > >[ 16.822058] #1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
    > >[ 16.822058]
    > >[ 16.822058] stack backtrace:
    > >[ 16.822058] CPU: 1 PID: 1198 Comm: bash Tainted: G W 3.11.0-rc3-next-20130730+ #140
    > >[ 16.822058] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
    > >[ 16.822058] 0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002
    > >[ 16.822058] 0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98
    > >[ 16.822058] ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670
    > >[ 16.822058] Call Trace:
    > >[ 16.822058] [<ffffffff81d34074>] dump_stack+0x59/0x7d
    > >[ 16.822058] [<ffffffff8110ed75>] print_circular_bug+0x105/0x120
    > >[ 16.822058] [<ffffffff8111064f>] check_prev_add+0x14f/0x590
    > >[ 16.822058] [<ffffffff81d3ab5f>] ? _raw_spin_unlock_irq+0x4f/0x70
    > >[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
    > >[ 16.822058] [<ffffffff8110ae0f>] ? trace_hardirqs_off_caller+0x1f/0x190
    > >[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
    > >[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
    > >[ 16.822058] [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
    > >[ 16.822058] [<ffffffff81d372c1>] down_read+0x51/0xa0
    > >[ 16.822058] [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
    > >[ 16.822058] [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
    > >[ 16.822058] [<ffffffff810e4130>] ? try_to_wake_up+0x210/0x210
    > >[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
    > >[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
    > >[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
    > >[ 16.822058] [<ffffffff815e24ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    > >[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
    > >
    > >Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
    > >---
    > > drivers/tty/n_tty.c | 12 ++++++++++++
    > > 1 files changed, 12 insertions(+), 0 deletions(-)
    > >
    > >diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
    > >index dd8ae0c..38c09db 100644
    > >--- a/drivers/tty/n_tty.c
    > >+++ b/drivers/tty/n_tty.c
    > >@@ -2203,11 +2203,23 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
    > > break;
    > > }
    > > n_tty_set_room(tty);
    > >+ mutex_unlock(&ldata->atomic_read_lock);
    > > up_read(&tty->termios_rwsem);
    > >
    > > timeout = schedule_timeout(timeout);
    > >
    > > down_read(&tty->termios_rwsem);
    > >+ if (file->f_flags & O_NONBLOCK) {
    > >+ if (!mutex_trylock(&ldata->atomic_read_lock)) {
    > >+ retval = -EAGAIN;
    > >+ break;
    > >+ }
    > >+ } else {
    > >+ if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
    > >+ retval = -ERESTARTSYS;
    > >+ break;
    > >+ }
    > >+ }
    > > continue;
    > > }
    > > __set_current_state(TASK_RUNNING);
    > >
    >

    --
    Regards,
    Artem


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