Although the test loop does randomly delay, which would provide quiescent
states and so forth, it is possible for there to be a series of long
smp_call_function*() handler runtimes with no delays, which results in
softlockup and RCU CPU stall warning messages.  This commit therefore
inserts a cond_resched() into the main test loop.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

On uniprocessor systems, smp_call_function() does nothing.  This commit
therefore avoids complaining about the lack of handler accesses in the
single-CPU case where there is no handler.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Currently, CPU-hotplug operations might result in all but two
of (say) 100 CPUs being offline, which in turn might result in
false-positive diagnostics due to overload.  This commit therefore
causes scftorture_invoker() kthreads for offline CPUs to loop blocking
for 200 milliseconds at a time, thus continuously adjusting the number
of threads to match the number of online CPUs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit adds a "default" case to the switch statement in
scftorture_invoke_one() which contains a WARN_ON_ONCE() and an assignment
to ->scfc_out to suppress knock-on warnings.  These knock-on warnings
could otherwise cause the user to think that there was a memory-ordering
problem in smp_call_function() instead of a bug in scftorture.c itself.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

The sparse tool complains as follows

kernel/scftorture.c:124:1: warning:
 symbol '__pcpu_scope_scf_torture_rand' was not declared. Should it be static?

And this per-CPU variable is not used outside of scftorture.c,
so this commit marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Detecting smp_call_function() memory misordering requires close timing,
so it is necessary to have the checks immediately before and after
the call to the smp_call_function*() function under test.  This commit
therefore inserts barrier() calls to prevent the compiler from optimizing
memory-misordering detection down into the zone of extreme improbability.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit prints error counts on the statistics line and also adds a
"!!!" if any of the counters are non-zero.  Allocation failures are
(somewhat) forgiven, but all other errors result in a "FAILURE" print
at the end of the test.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit hoists much of the initialization of the scf_check
structure out of the switch statement, thus saving a few lines of code.
The initialization of the ->scfc_in field remains in each leg of the
switch statement in order to more heavily stress memory ordering.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit moves checking of the ->scfc_out field and the freeing of
the scf_check structure down below the end of switch statement, thus
saving a few lines of code.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

This commit adds checks for memory misordering across calls to and
returns from smp_call_function() in the case where the caller waits.
Misordering results in a splat.

Note that in contrast to smp_call_function_single(), this code does not
test memory ordering into the handler in the no-wait case because none
of the handlers would be able to free the scf_check structure without
introducing heavy synchronization to work out which was last.

[ paulmck: s/GFP_KERNEL/GFP_ATOMIC/ per kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>