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Message-Id: <20200623000947.25222-2-paulmck@kernel.org>
Date: Mon, 22 Jun 2020 17:09:39 -0700
From: paulmck@...nel.org
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 02/10] docs: RCU: Convert lockdep-splat.txt to ReST
From: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
- 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@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
Documentation/RCU/index.rst | 1 +
.../RCU/{lockdep-splat.txt => lockdep-splat.rst} | 109 +++++++++++----------
2 files changed, 58 insertions(+), 52 deletions(-)
rename Documentation/RCU/{lockdep-splat.txt => lockdep-splat.rst} (54%)
diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index c1ba4d1..430a371 100644
--- a/Documentation/RCU/index.rst
+++ b/Documentation/RCU/index.rst
@@ -11,6 +11,7 @@ RCU concepts
arrayRCU
checklist
+ lockdep-splat
rcubarrier
rcu_dereference
whatisRCU
diff --git a/Documentation/RCU/lockdep-splat.txt b/Documentation/RCU/lockdep-splat.rst
similarity index 54%
rename from Documentation/RCU/lockdep-splat.txt
rename to Documentation/RCU/lockdep-splat.rst
index b809631..2a5c79d 100644
--- a/Documentation/RCU/lockdep-splat.txt
+++ b/Documentation/RCU/lockdep-splat.rst
@@ -1,3 +1,9 @@
+.. 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()
@@ -12,55 +18,54 @@ 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:
+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) {
@@ -70,7 +75,7 @@ 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:
+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) {
@@ -85,7 +90,7 @@ 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:
+add rcu_read_lock() and rcu_read_unlock() as follows::
rcu_read_lock();
if (rcu_dereference(ioc->ioc_data) == cic) {
@@ -102,7 +107,7 @@ 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:
+by rcu_access_pointer() as follows::
if (rcu_access_pointer(ioc->ioc_data) == cic) {
--
2.9.5
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