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Subject[tip: locking/urgent] kcsan: Document "value changed" line
The following commit has been merged into the locking/urgent branch of tip:

Commit-ID: b930226f3db870cfb683c2744aeb0d29deb4cddc
Gitweb: https://git.kernel.org/tip/b930226f3db870cfb683c2744aeb0d29deb4cddc
Author: Marco Elver <elver@google.com>
AuthorDate: Wed, 14 Apr 2021 13:28:25 +02:00
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Tue, 18 May 2021 10:58:15 -07:00

kcsan: Document "value changed" line

Update the example reports based on the latest reports generated by
kcsan_test module, which now include the "value changed" line. Add a
brief description of the "value changed" line.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
Documentation/dev-tools/kcsan.rst | 88 +++++++++++-------------------
1 file changed, 35 insertions(+), 53 deletions(-)

diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
index 8089466..d1efd9c 100644
--- a/Documentation/dev-tools/kcsan.rst
+++ b/Documentation/dev-tools/kcsan.rst
@@ -27,75 +27,57 @@ Error reports
A typical data race report looks like this::

==================================================================
- BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
-
- write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
- kernfs_refresh_inode+0x70/0x170
- kernfs_iop_permission+0x4f/0x90
- inode_permission+0x190/0x200
- link_path_walk.part.0+0x503/0x8e0
- path_lookupat.isra.0+0x69/0x4d0
- filename_lookup+0x136/0x280
- user_path_at_empty+0x47/0x60
- vfs_statx+0x9b/0x130
- __do_sys_newlstat+0x50/0xb0
- __x64_sys_newlstat+0x37/0x50
- do_syscall_64+0x85/0x260
- entry_SYSCALL_64_after_hwframe+0x44/0xa9
-
- read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
- generic_permission+0x5b/0x2a0
- kernfs_iop_permission+0x66/0x90
- inode_permission+0x190/0x200
- link_path_walk.part.0+0x503/0x8e0
- path_lookupat.isra.0+0x69/0x4d0
- filename_lookup+0x136/0x280
- user_path_at_empty+0x47/0x60
- do_faccessat+0x11a/0x390
- __x64_sys_access+0x3c/0x50
- do_syscall_64+0x85/0x260
- entry_SYSCALL_64_after_hwframe+0x44/0xa9
+ BUG: KCSAN: data-race in test_kernel_read / test_kernel_write
+
+ write to 0xffffffffc009a628 of 8 bytes by task 487 on cpu 0:
+ test_kernel_write+0x1d/0x30
+ access_thread+0x89/0xd0
+ kthread+0x23e/0x260
+ ret_from_fork+0x22/0x30
+
+ read to 0xffffffffc009a628 of 8 bytes by task 488 on cpu 6:
+ test_kernel_read+0x10/0x20
+ access_thread+0x89/0xd0
+ kthread+0x23e/0x260
+ ret_from_fork+0x22/0x30
+
+ value changed: 0x00000000000009a6 -> 0x00000000000009b2

Reported by Kernel Concurrency Sanitizer on:
- CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
- Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+ CPU: 6 PID: 488 Comm: access_thread Not tainted 5.12.0-rc2+ #1
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
==================================================================

The header of the report provides a short summary of the functions involved in
the race. It is followed by the access types and stack traces of the 2 threads
-involved in the data race.
+involved in the data race. If KCSAN also observed a value change, the observed
+old value and new value are shown on the "value changed" line respectively.

The other less common type of data race report looks like this::

==================================================================
- BUG: KCSAN: data-race in e1000_clean_rx_irq+0x551/0xb10
-
- race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
- e1000_clean_rx_irq+0x551/0xb10
- e1000_clean+0x533/0xda0
- net_rx_action+0x329/0x900
- __do_softirq+0xdb/0x2db
- irq_exit+0x9b/0xa0
- do_IRQ+0x9c/0xf0
- ret_from_intr+0x0/0x18
- default_idle+0x3f/0x220
- arch_cpu_idle+0x21/0x30
- do_idle+0x1df/0x230
- cpu_startup_entry+0x14/0x20
- rest_init+0xc5/0xcb
- arch_call_rest_init+0x13/0x2b
- start_kernel+0x6db/0x700
+ BUG: KCSAN: data-race in test_kernel_rmw_array+0x71/0xd0
+
+ race at unknown origin, with read to 0xffffffffc009bdb0 of 8 bytes by task 515 on cpu 2:
+ test_kernel_rmw_array+0x71/0xd0
+ access_thread+0x89/0xd0
+ kthread+0x23e/0x260
+ ret_from_fork+0x22/0x30
+
+ value changed: 0x0000000000002328 -> 0x0000000000002329

Reported by Kernel Concurrency Sanitizer on:
- CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
- Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+ CPU: 2 PID: 515 Comm: access_thread Not tainted 5.12.0-rc2+ #1
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
==================================================================

This report is generated where it was not possible to determine the other
racing thread, but a race was inferred due to the data value of the watched
-memory location having changed. These can occur either due to missing
-instrumentation or e.g. DMA accesses. These reports will only be generated if
-``CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=y`` (selected by default).
+memory location having changed. These reports always show a "value changed"
+line. A common reason for reports of this type are missing instrumentation in
+the racing thread, but could also occur due to e.g. DMA accesses. Such reports
+are shown only if ``CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=y``, which is
+enabled by default.

Selective analysis
~~~~~~~~~~~~~~~~~~
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