path: root/Documentation
diff options
authorHidetoshi Seto <>2009-07-29 15:04:14 -0700
committerLinus Torvalds <>2009-07-29 19:10:36 -0700
commitcab8bd3410d448279e3bd0fbf96d31db0bf770fa (patch)
tree2ad9262b2c4d10cc705c8553015caff06a29de11 /Documentation
parentf5a55efa140f5e9c9dd0f398fef54f20cdb74ec9 (diff)
sysrq, kdump: make sysrq-c consistent
commit d6580a9f15238b87e618310c862231ae3f352d2d ("kexec: sysrq: simplify sysrq-c handler") changed the behavior of sysrq-c to unconditional dereference of NULL pointer. So in cases with CONFIG_KEXEC, where crash_kexec() was directly called from sysrq-c before, now it can be said that a step of "real oops" was inserted before starting kdump. However, in contrast to oops via SysRq-c from keyboard which results in panic due to in_interrupt(), oops via "echo c > /proc/sysrq-trigger" will not become panic unless panic_on_oops=1. It means that even if dump is properly configured to be taken on panic, the sysrq-c from proc interface might not start crashdump while the sysrq-c from keyboard can start crashdump. This confuses traditional users of kdump, i.e. people who expect sysrq-c to do common behavior in both of the keyboard and proc interface. This patch brings the keyboard and proc interface behavior of sysrq-c in line, by forcing panic_on_oops=1 before oops in sysrq-c handler. And some updates in documentation are included, to clarify that there is no longer dependency with CONFIG_KEXEC, and that now the system can just crash by sysrq-c if no dump mechanism is configured. Signed-off-by: Hidetoshi Seto <> Cc: Lai Jiangshan <> Cc: Ken'ichi Ohmichi <> Acked-by: Neil Horman <> Acked-by: Vivek Goyal <> Cc: Brayan Arraes <> Cc: Eric W. Biederman <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation')
1 files changed, 4 insertions, 3 deletions
diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
index cf42b820ff9d..d56a01775423 100644
--- a/Documentation/sysrq.txt
+++ b/Documentation/sysrq.txt
@@ -66,7 +66,8 @@ On all - write a character to /proc/sysrq-trigger. e.g.:
'b' - Will immediately reboot the system without syncing or unmounting
your disks.
-'c' - Will perform a kexec reboot in order to take a crashdump.
+'c' - Will perform a system crash by a NULL pointer dereference.
+ A crashdump will be taken if configured.
'd' - Shows all locks that are held.
@@ -141,8 +142,8 @@ useful when you want to exit a program that will not let you switch consoles.
re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.
-'C'rashdump can be used to manually trigger a crashdump when the system is hung.
-The kernel needs to have been built with CONFIG_KEXEC enabled.
+'C'rash can be used to manually trigger a crashdump when the system is hung.
+Note that this just triggers a crash if there is no dump mechanism available.
'S'ync is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note