1.. SPDX-License-Identifier: GPL-2.0 2 3================= 4Lockdep-RCU Splat 5================= 6 7Lockdep-RCU was added to the Linux kernel in early 2010 8(http://lwn.net/Articles/371986/). This facility checks for some common 9misuses of the RCU API, most notably using one of the rcu_dereference() 10family to access an RCU-protected pointer without the proper protection. 11When such misuse is detected, an lockdep-RCU splat is emitted. 12 13The usual cause of a lockdep-RCU slat is someone accessing an 14RCU-protected data structure without either (1) being in the right kind of 15RCU read-side critical section or (2) holding the right update-side lock. 16This problem can therefore be serious: it might result in random memory 17overwriting or worse. There can of course be false positives, this 18being the real world and all that. 19 20So let's look at an example RCU lockdep splat from 3.0-rc5, one that 21has long since been fixed:: 22 23 ============================= 24 WARNING: suspicious RCU usage 25 ----------------------------- 26 block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage! 27 28other info that might help us debug this:: 29 30 rcu_scheduler_active = 1, debug_locks = 0 31 3 locks held by scsi_scan_6/1552: 32 #0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>] 33 scsi_scan_host_selected+0x5a/0x150 34 #1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>] 35 elevator_exit+0x22/0x60 36 #2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>] 37 cfq_exit_queue+0x43/0x190 38 39 stack backtrace: 40 Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17 41 Call Trace: 42 [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0 43 [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120 44 [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190 45 [<ffffffff812a5046>] elevator_exit+0x36/0x60 46 [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60 47 [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10 48 [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0 49 [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10 50 [<ffffffff817da069>] ? error_exit+0x29/0xb0 51 [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80 52 [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680 53 [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c 54 [<ffffffff817da069>] ? error_exit+0x29/0xb0 55 [<ffffffff812bcc60>] ? kobject_del+0x40/0x40 56 [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0 57 [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150 58 [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90 59 [<ffffffff8145f170>] do_scan_async+0x20/0x160 60 [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90 61 [<ffffffff810975b6>] kthread+0xa6/0xb0 62 [<ffffffff817db154>] kernel_thread_helper+0x4/0x10 63 [<ffffffff81066430>] ? finish_task_switch+0x80/0x110 64 [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe 65 [<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70 66 [<ffffffff817db150>] ? gs_change+0xb/0xb 67 68Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:: 69 70 if (rcu_dereference(ioc->ioc_data) == cic) { 71 72This form says that it must be in a plain vanilla RCU read-side critical 73section, but the "other info" list above shows that this is not the 74case. Instead, we hold three locks, one of which might be RCU related. 75And maybe that lock really does protect this reference. If so, the fix 76is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to 77take the struct request_queue "q" from cfq_exit_queue() as an argument, 78which would permit us to invoke rcu_dereference_protected as follows:: 79 80 if (rcu_dereference_protected(ioc->ioc_data, 81 lockdep_is_held(&q->queue_lock)) == cic) { 82 83With this change, there would be no lockdep-RCU splat emitted if this 84code was invoked either from within an RCU read-side critical section 85or with the ->queue_lock held. In particular, this would have suppressed 86the above lockdep-RCU splat because ->queue_lock is held (see #2 in the 87list above). 88 89On the other hand, perhaps we really do need an RCU read-side critical 90section. In this case, the critical section must span the use of the 91return value from rcu_dereference(), or at least until there is some 92reference count incremented or some such. One way to handle this is to 93add rcu_read_lock() and rcu_read_unlock() as follows:: 94 95 rcu_read_lock(); 96 if (rcu_dereference(ioc->ioc_data) == cic) { 97 spin_lock(&ioc->lock); 98 rcu_assign_pointer(ioc->ioc_data, NULL); 99 spin_unlock(&ioc->lock); 100 } 101 rcu_read_unlock(); 102 103With this change, the rcu_dereference() is always within an RCU 104read-side critical section, which again would have suppressed the 105above lockdep-RCU splat. 106 107But in this particular case, we don't actually dereference the pointer 108returned from rcu_dereference(). Instead, that pointer is just compared 109to the cic pointer, which means that the rcu_dereference() can be replaced 110by rcu_access_pointer() as follows:: 111 112 if (rcu_access_pointer(ioc->ioc_data) == cic) { 113 114Because it is legal to invoke rcu_access_pointer() without protection, 115this change would also suppress the above lockdep-RCU splat. 116