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When running ktest.pl randconfig tests, I would sometimes trigger
a lockdep annotation bug (possible reason: unannotated irqs-on).
This triggering happened right after function tracer self test was
executed. After doing a config bisect I found that this was caused with
having function tracer, paravirt guest, prove locking, and rcu torture
all enabled.
The rcu torture just enhanced the likelyhood of triggering the bug.
Prove locking was needed, since it was the thing that was bugging.
Function tracer would trace and disable interrupts in all sorts
of funny places.
paravirt guest would turn arch_local_irq_* into functions that would
be traced.
Besides the fact that tracing arch_local_irq_* is just a bad idea,
this is what is happening.
The bug happened simply in the local_irq_restore() code:
if (raw_irqs_disabled_flags(flags)) { \
raw_local_irq_restore(flags); \
trace_hardirqs_off(); \
} else { \
trace_hardirqs_on(); \
raw_local_irq_restore(flags); \
} \
The raw_local_irq_restore() was defined as arch_local_irq_restore().
Now imagine, we are about to enable interrupts. We go into the else
case and call trace_hardirqs_on() which tells lockdep that we are enabling
interrupts, so it sets the current->hardirqs_enabled = 1.
Then we call raw_local_irq_restore() which calls arch_local_irq_restore()
which gets traced!
Now in the function tracer we disable interrupts with local_irq_save().
This is fine, but flags is stored that we have interrupts disabled.
When the function tracer calls local_irq_restore() it does it, but this
time with flags set as disabled, so we go into the if () path.
This keeps interrupts disabled and calls trace_hardirqs_off() which
sets current->hardirqs_enabled = 0.
When the tracer is finished and proceeds with the original code,
we enable interrupts but leave current->hardirqs_enabled as 0. Which
now breaks lockdeps internal processing.
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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