docs: RCU: Convert lockdep-splat.txt to ReST
- Add a SPDX header; - Add a document title; - Some whitespace fixes and new line breaks; - Mark literal blocks as such; - Add it to RCU/index.rst. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Mauro Carvalho Chehab
Paul E. McKenney
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6b05dfacd7
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a3b0a79f89
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@ -11,6 +11,7 @@ RCU concepts
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arrayRCU
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checklist
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lockdep-splat
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rcubarrier
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rcu_dereference
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whatisRCU
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@ -1,3 +1,9 @@
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.. SPDX-License-Identifier: GPL-2.0
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=================
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Lockdep-RCU Splat
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=================
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Lockdep-RCU was added to the Linux kernel in early 2010
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(http://lwn.net/Articles/371986/). This facility checks for some common
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misuses of the RCU API, most notably using one of the rcu_dereference()
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@ -12,55 +18,54 @@ overwriting or worse. There can of course be false positives, this
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being the real world and all that.
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So let's look at an example RCU lockdep splat from 3.0-rc5, one that
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has long since been fixed:
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has long since been fixed::
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=============================
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WARNING: suspicious RCU usage
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-----------------------------
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block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage!
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=============================
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WARNING: suspicious RCU usage
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-----------------------------
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block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage!
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other info that might help us debug this:
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other info that might help us debug this::
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rcu_scheduler_active = 1, debug_locks = 0
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3 locks held by scsi_scan_6/1552:
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#0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>]
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scsi_scan_host_selected+0x5a/0x150
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#1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>]
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elevator_exit+0x22/0x60
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#2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>]
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cfq_exit_queue+0x43/0x190
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rcu_scheduler_active = 1, debug_locks = 0
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3 locks held by scsi_scan_6/1552:
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#0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>]
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scsi_scan_host_selected+0x5a/0x150
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#1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>]
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elevator_exit+0x22/0x60
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#2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>]
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cfq_exit_queue+0x43/0x190
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stack backtrace:
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Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17
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Call Trace:
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[<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0
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[<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120
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[<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190
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[<ffffffff812a5046>] elevator_exit+0x36/0x60
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[<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60
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[<ffffffff8145cc09>] scsi_free_queue+0x9/0x10
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[<ffffffff81460944>] __scsi_remove_device+0x84/0xd0
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[<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10
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[<ffffffff817da069>] ? error_exit+0x29/0xb0
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[<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
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[<ffffffff8145e722>] __scsi_scan_target+0x112/0x680
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[<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c
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[<ffffffff817da069>] ? error_exit+0x29/0xb0
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[<ffffffff812bcc60>] ? kobject_del+0x40/0x40
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[<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0
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[<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150
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[<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90
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[<ffffffff8145f170>] do_scan_async+0x20/0x160
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[<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90
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[<ffffffff810975b6>] kthread+0xa6/0xb0
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[<ffffffff817db154>] kernel_thread_helper+0x4/0x10
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[<ffffffff81066430>] ? finish_task_switch+0x80/0x110
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[<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe
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[<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70
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[<ffffffff817db150>] ? gs_change+0xb/0xb
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stack backtrace:
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Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17
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Call Trace:
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[<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0
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[<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120
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[<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190
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[<ffffffff812a5046>] elevator_exit+0x36/0x60
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[<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60
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[<ffffffff8145cc09>] scsi_free_queue+0x9/0x10
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[<ffffffff81460944>] __scsi_remove_device+0x84/0xd0
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[<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10
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[<ffffffff817da069>] ? error_exit+0x29/0xb0
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[<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
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[<ffffffff8145e722>] __scsi_scan_target+0x112/0x680
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[<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c
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[<ffffffff817da069>] ? error_exit+0x29/0xb0
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[<ffffffff812bcc60>] ? kobject_del+0x40/0x40
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[<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0
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[<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150
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[<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90
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[<ffffffff8145f170>] do_scan_async+0x20/0x160
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[<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90
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[<ffffffff810975b6>] kthread+0xa6/0xb0
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[<ffffffff817db154>] kernel_thread_helper+0x4/0x10
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[<ffffffff81066430>] ? finish_task_switch+0x80/0x110
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[<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe
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[<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70
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[<ffffffff817db150>] ? gs_change+0xb/0xb
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Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:
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Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows::
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if (rcu_dereference(ioc->ioc_data) == cic) {
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@ -70,7 +75,7 @@ case. Instead, we hold three locks, one of which might be RCU related.
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And maybe that lock really does protect this reference. If so, the fix
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is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to
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take the struct request_queue "q" from cfq_exit_queue() as an argument,
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which would permit us to invoke rcu_dereference_protected as follows:
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which would permit us to invoke rcu_dereference_protected as follows::
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if (rcu_dereference_protected(ioc->ioc_data,
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lockdep_is_held(&q->queue_lock)) == cic) {
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@ -85,7 +90,7 @@ On the other hand, perhaps we really do need an RCU read-side critical
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section. In this case, the critical section must span the use of the
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return value from rcu_dereference(), or at least until there is some
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reference count incremented or some such. One way to handle this is to
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add rcu_read_lock() and rcu_read_unlock() as follows:
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add rcu_read_lock() and rcu_read_unlock() as follows::
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rcu_read_lock();
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if (rcu_dereference(ioc->ioc_data) == cic) {
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@ -102,7 +107,7 @@ above lockdep-RCU splat.
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But in this particular case, we don't actually dereference the pointer
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returned from rcu_dereference(). Instead, that pointer is just compared
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to the cic pointer, which means that the rcu_dereference() can be replaced
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by rcu_access_pointer() as follows:
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by rcu_access_pointer() as follows::
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if (rcu_access_pointer(ioc->ioc_data) == cic) {
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