![Paul E. McKenney](/assets/img/avatar_default.png)
Uses of RCU for dynamically changeable NMI handlers need to use the new rcu_dereference() and rcu_assign_pointer() facilities. This change makes it clear that these uses are safe from a memory-barrier viewpoint, but the main purpose is to document exactly what operations are being protected by RCU. This has been tested on x86 and x86-64, which are the only architectures affected by this change. Signed-off-by: <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
113 lines
3.8 KiB
Text
113 lines
3.8 KiB
Text
Using RCU to Protect Dynamic NMI Handlers
|
|
|
|
|
|
Although RCU is usually used to protect read-mostly data structures,
|
|
it is possible to use RCU to provide dynamic non-maskable interrupt
|
|
handlers, as well as dynamic irq handlers. This document describes
|
|
how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer
|
|
work in "arch/i386/oprofile/nmi_timer_int.c" and in
|
|
"arch/i386/kernel/traps.c".
|
|
|
|
The relevant pieces of code are listed below, each followed by a
|
|
brief explanation.
|
|
|
|
static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
The dummy_nmi_callback() function is a "dummy" NMI handler that does
|
|
nothing, but returns zero, thus saying that it did nothing, allowing
|
|
the NMI handler to take the default machine-specific action.
|
|
|
|
static nmi_callback_t nmi_callback = dummy_nmi_callback;
|
|
|
|
This nmi_callback variable is a global function pointer to the current
|
|
NMI handler.
|
|
|
|
fastcall void do_nmi(struct pt_regs * regs, long error_code)
|
|
{
|
|
int cpu;
|
|
|
|
nmi_enter();
|
|
|
|
cpu = smp_processor_id();
|
|
++nmi_count(cpu);
|
|
|
|
if (!rcu_dereference(nmi_callback)(regs, cpu))
|
|
default_do_nmi(regs);
|
|
|
|
nmi_exit();
|
|
}
|
|
|
|
The do_nmi() function processes each NMI. It first disables preemption
|
|
in the same way that a hardware irq would, then increments the per-CPU
|
|
count of NMIs. It then invokes the NMI handler stored in the nmi_callback
|
|
function pointer. If this handler returns zero, do_nmi() invokes the
|
|
default_do_nmi() function to handle a machine-specific NMI. Finally,
|
|
preemption is restored.
|
|
|
|
Strictly speaking, rcu_dereference() is not needed, since this code runs
|
|
only on i386, which does not need rcu_dereference() anyway. However,
|
|
it is a good documentation aid, particularly for anyone attempting to
|
|
do something similar on Alpha.
|
|
|
|
Quick Quiz: Why might the rcu_dereference() be necessary on Alpha,
|
|
given that the code referenced by the pointer is read-only?
|
|
|
|
|
|
Back to the discussion of NMI and RCU...
|
|
|
|
void set_nmi_callback(nmi_callback_t callback)
|
|
{
|
|
rcu_assign_pointer(nmi_callback, callback);
|
|
}
|
|
|
|
The set_nmi_callback() function registers an NMI handler. Note that any
|
|
data that is to be used by the callback must be initialized up -before-
|
|
the call to set_nmi_callback(). On architectures that do not order
|
|
writes, the rcu_assign_pointer() ensures that the NMI handler sees the
|
|
initialized values.
|
|
|
|
void unset_nmi_callback(void)
|
|
{
|
|
rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
|
|
}
|
|
|
|
This function unregisters an NMI handler, restoring the original
|
|
dummy_nmi_handler(). However, there may well be an NMI handler
|
|
currently executing on some other CPU. We therefore cannot free
|
|
up any data structures used by the old NMI handler until execution
|
|
of it completes on all other CPUs.
|
|
|
|
One way to accomplish this is via synchronize_sched(), perhaps as
|
|
follows:
|
|
|
|
unset_nmi_callback();
|
|
synchronize_sched();
|
|
kfree(my_nmi_data);
|
|
|
|
This works because synchronize_sched() blocks until all CPUs complete
|
|
any preemption-disabled segments of code that they were executing.
|
|
Since NMI handlers disable preemption, synchronize_sched() is guaranteed
|
|
not to return until all ongoing NMI handlers exit. It is therefore safe
|
|
to free up the handler's data as soon as synchronize_sched() returns.
|
|
|
|
|
|
Answer to Quick Quiz
|
|
|
|
Why might the rcu_dereference() be necessary on Alpha, given
|
|
that the code referenced by the pointer is read-only?
|
|
|
|
Answer: The caller to set_nmi_callback() might well have
|
|
initialized some data that is to be used by the
|
|
new NMI handler. In this case, the rcu_dereference()
|
|
would be needed, because otherwise a CPU that received
|
|
an NMI just after the new handler was set might see
|
|
the pointer to the new NMI handler, but the old
|
|
pre-initialized version of the handler's data.
|
|
|
|
More important, the rcu_dereference() makes it clear
|
|
to someone reading the code that the pointer is being
|
|
protected by RCU.
|