Linus,
During testing Sedat Dilek hit a "suspicious RCU usage" splat that pointed
out a real bug. During suspend and resume the tlb_flush tracepoint is
called when the CPU is going offline. As the CPU has been noted as offline,
RCU is ignoring that CPU, which means that it can not use RCU protected
locks. When tracepoints are activated, they require RCU locking, and
if RCU is ignoring a CPU that runs a tracepoint, there is a chance that
the tracepoint could cause corruption.
The solution was to change the tracepoint into a TRACE_EVENT_CONDITION()
which allows us to check a condition to determine if the tracepoint
should be called or not. If the condition is not met, the rcu protected
code will not be executed. By adding the condition
"cpu_online(smp_processor_id())", this will prevent the RCU protected
code from being executed if the CPU is marked offline.
After adding this, another bug was discovered. As RCU checks rcu callers,
if a rcu call is not done, there is no check (obviously). We found that
tracepoints could be added in RCU ignored locations and not have lockdep
complain until the tracepoint is activated. This missed places where
tracepoints were added in places they should not have been. To fix this,
code was added in 3.18 that if lockdep is enabled, any tracepoint will
still call the rcu checks even if the tracepoint is not enabled. The bug
here, is that the check does not take the CONDITION into account. As the
condition may prevent tracepoints from being activated in RCU ignored
areas (as the one patch does), we get false positives when we enable
lockdep and hit a tracepoint that the condition prevents it from being
called in a RCU ignored location. The fix for this is to add the
CONDITION to the rcu checks, even if the tracepoint is not enabled.
Please pull the latest trace-fixes-v3.19-rc7 tree, which can be found at:
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
trace-fixes-v3.19-rc7
Tag SHA1: 8c1c683d0b8af59da6b65976bb7f2a2f1cac7345
Head SHA1: 6c8465a82a605bc692304bab42703017dcfff013
Steven Rostedt (Red Hat) (2):
tracing: Add condition check to RCU lockdep checks
x86/tlb/trace: Do not trace on CPU that is offline
----
include/linux/tracepoint.h | 2 +-
include/trace/events/tlb.h | 4 +++-
2 files changed, 4 insertions(+), 2 deletions(-)
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