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.
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Merge tag 'trace-fixes-v3.19-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace
Pull ftrace fixes from Steven Rostedt:
"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"
* tag 'trace-fixes-v3.19-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
x86/tlb/trace: Do not trace on CPU that is offline
tracing: Add condition check to RCU lockdep checks
4e02370f64