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 <mathieu.desnoyers@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20241203163558.3455535-1-mathieu.desnoyers@efficios.com

diff --git a/kernel/smp.c b/kernel/smp.c
index 27dc31a146a3599564e46ca55766c7e9dca8ca35..f104c8e83fc4c4a0b0de948ea2cf3bb5e6349dd3 100644 (file)
--- a/kernel/smp.c
+++ b/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);