path: root/Documentation/RCU/lockdep-splat.txt
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-Lockdep-RCU was added to the Linux kernel in early 2010
-( 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>]
- #1: (&eq->sysfs_lock){+.+...}, at: [<ffffffff812a5032>]
- #2: (&(&q->__queue_lock)->rlock){-.-...}, at: [<ffffffff812b6233>]
-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.