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| -rw-r--r-- | Documentation/RCU/lockdep-splat.rst | 115 |
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diff --git a/Documentation/RCU/lockdep-splat.rst b/Documentation/RCU/lockdep-splat.rst new file mode 100644 index 000000000..2a5c79db5 --- /dev/null +++ b/Documentation/RCU/lockdep-splat.rst @@ -0,0 +1,115 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================= +Lockdep-RCU Splat +================= + +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:: + + ============================= + WARNING: 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>] ? __kthread_init_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 dereference 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. |