smp/scf: Evaluate local cond_func() before IPI side-effects

In smp_call_function_many_cond(), the local cond_func() is evaluated
after triggering the remote CPU IPIs.

If cond_func() depends on loading shared state updated by other CPU's
IPI handlers func(), then triggering execution of remote CPUs IPI before
evaluating cond_func() may have unexpected consequences.

One example scenario is evaluating a jiffies delay in cond_func(), which
is updated by func() in the IPI handlers. This situation can prevent
execution of periodic cleanup code on the local CPU.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: compudj

kernel/smp.c

@@ -815,7 +815,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 	WARN_ON_ONCE(!in_task());
 
 	/* Check if we need local execution. */
-	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask))
+	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask) &&
+	    (!cond_func || cond_func(this_cpu, info)))
 		run_local = true;
 
 	/* Check if we need remote execution, i.e., any CPU excluding this one. */
@@ -868,7 +869,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 			send_call_function_ipi_mask(cfd->cpumask_ipi);
 	}
 
-	if (run_local && (!cond_func || cond_func(this_cpu, info))) {
+	if (run_local) {
 		unsigned long flags;
 
 		local_irq_save(flags);