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>

1 files changed, 115 insertions, 0 deletions

diff --git a/Documentation/RCU/lockdep-splat.rst b/Documentation/RCU/lockdep-splat.rst
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+.. 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.