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kcsan: Document "value changed" line

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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>
This commit is contained in:
Marco Elver 2021-04-14 13:28:25 +02:00 committed by Paul E. McKenney
parent 7bbe6dc0ad
commit b930226f3d
1 changed files with 32 additions and 50 deletions
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82
Documentation/dev-tools/kcsan.rst
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@ -27,75 +27,57 @@ Error reports
A typical data race report looks like this::
==================================================================
BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
BUG: KCSAN: data-race in test_kernel_read / test_kernel_write
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
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 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
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
BUG: KCSAN: data-race in test_kernel_rmw_array+0x71/0xd0
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
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
~~~~~~~~~~~~~~~~~~