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    Subject[PATCH tip/core/rcu 28/55] rcu: Document interpretation of RCU-lockdep splats
    Date
    There has been quite a bit of confusion about what RCU-lockdep splats
    mean, so this commit adds some documentation describing how to
    interpret them.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    Documentation/RCU/lockdep-splat.txt | 110 +++++++++++++++++++++++++++++++++++
    1 files changed, 110 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/RCU/lockdep-splat.txt

    diff --git a/Documentation/RCU/lockdep-splat.txt b/Documentation/RCU/lockdep-splat.txt
    new file mode 100644
    index 0000000..bf90611
    --- /dev/null
    +++ b/Documentation/RCU/lockdep-splat.txt
    @@ -0,0 +1,110 @@
    +Lockdep-RCU was added to the Linux kernel in early 2010
    +(http://lwn.net/Articles/371986/). This facility checks for some common
    +misuses of the RCU API, most notably using one of the rcu_dereference()
    +family to access an RCU-protected pointer without the proper protection.
    +When such misuse is detected, an lockdep-RCU splat is emitted.
    +
    +The usual cause of a lockdep-RCU slat is someone accessing an
    +RCU-protected data structure without either (1) being in the right kind of
    +RCU read-side critical section or (2) holding the right update-side lock.
    +This problem can therefore be serious: it might result in random memory
    +overwriting or worse. There can of course be false positives, this
    +being the real world and all that.
    +
    +So let's look at an example RCU lockdep splat from 3.0-rc5, one that
    +has long since been fixed:
    +
    +===============================
    +[ INFO: suspicious RCU usage. ]
    +-------------------------------
    +block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage!
    +
    +other info that might help us debug this:
    +
    +
    +rcu_scheduler_active = 1, debug_locks = 0
    +3 locks held by scsi_scan_6/1552:
    + #0: (&shost->scan_mutex){+.+.+.}, at: [<ffffffff8145efca>]
    +scsi_scan_host_selected+0x5a/0x150
    + #1: (&eq->sysfs_lock){+.+...}, at: [<ffffffff812a5032>]
    +elevator_exit+0x22/0x60
    + #2: (&(&q->__queue_lock)->rlock){-.-...}, at: [<ffffffff812b6233>]
    +cfq_exit_queue+0x43/0x190
    +
    +stack backtrace:
    +Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17
    +Call Trace:
    + [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0
    + [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120
    + [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190
    + [<ffffffff812a5046>] elevator_exit+0x36/0x60
    + [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60
    + [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10
    + [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0
    + [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10
    + [<ffffffff817da069>] ? error_exit+0x29/0xb0
    + [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
    + [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680
    + [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c
    + [<ffffffff817da069>] ? error_exit+0x29/0xb0
    + [<ffffffff812bcc60>] ? kobject_del+0x40/0x40
    + [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0
    + [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150
    + [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90
    + [<ffffffff8145f170>] do_scan_async+0x20/0x160
    + [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90
    + [<ffffffff810975b6>] kthread+0xa6/0xb0
    + [<ffffffff817db154>] kernel_thread_helper+0x4/0x10
    + [<ffffffff81066430>] ? finish_task_switch+0x80/0x110
    + [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe
    + [<ffffffff81097510>] ? __init_kthread_worker+0x70/0x70
    + [<ffffffff817db150>] ? gs_change+0xb/0xb
    +
    +Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:
    +
    + if (rcu_dereference(ioc->ioc_data) == cic) {
    +
    +This form says that it must be in a plain vanilla RCU read-side critical
    +section, but the "other info" list above shows that this is not the
    +case. Instead, we hold three locks, one of which might be RCU related.
    +And maybe that lock really does protect this reference. If so, the fix
    +is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to
    +take the struct request_queue "q" from cfq_exit_queue() as an argument,
    +which would permit us to invoke rcu_dereference_protected as follows:
    +
    + if (rcu_dereference_protected(ioc->ioc_data,
    + lockdep_is_held(&q->queue_lock)) == cic) {
    +
    +With this change, there would be no lockdep-RCU splat emitted if this
    +code was invoked either from within an RCU read-side critical section
    +or with the ->queue_lock held. In particular, this would have suppressed
    +the above lockdep-RCU splat because ->queue_lock is held (see #2 in the
    +list above).
    +
    +On the other hand, perhaps we really do need an RCU read-side critical
    +section. In this case, the critical section must span the use of the
    +return value from rcu_dereference(), or at least until there is some
    +reference count incremented or some such. One way to handle this is to
    +add rcu_read_lock() and rcu_read_unlock() as follows:
    +
    + rcu_read_lock();
    + if (rcu_dereference(ioc->ioc_data) == cic) {
    + spin_lock(&ioc->lock);
    + rcu_assign_pointer(ioc->ioc_data, NULL);
    + spin_unlock(&ioc->lock);
    + }
    + rcu_read_unlock();
    +
    +With this change, the rcu_dereference() is always within an RCU
    +read-side critical section, which again would have suppressed the
    +above lockdep-RCU splat.
    +
    +But in this particular case, we don't actually deference the pointer
    +returned from rcu_dereference(). Instead, that pointer is just compared
    +to the cic pointer, which means that the rcu_dereference() can be replaced
    +by rcu_access_pointer() as follows:
    +
    + if (rcu_access_pointer(ioc->ioc_data) == cic) {
    +
    +Because it is legal to invoke rcu_access_pointer() without protection,
    +this change would also suppress the above lockdep-RCU splat.
    --
    1.7.3.2


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