Re: [PATCH] smp: prevent soft lockup in smp_call_function_many_cond

On Tue, Jun 02, 2026 at 09:18:39PM +0000, Chris Packham wrote:

(adding others suggested by get_maintainer.pl)

On 27/05/2026 15:16, Mark Tomlinson wrote:

quoted

Using the PowerPC P2040 (e500mc) CPU, soft lockups can occasionally be
seen in smp_call_function_many_cond(). The conclusion is that this CPU
does not process the doorbell interrupt while in a data-storage (MMU)
exception. If more than one CPU in a multi core environment is calling
this function at the same time, it is possible for a deadlock to occur.

The fix for this is to call flush_smp_call_function_queue() before
waiting for responses from other CPUs. If there is something in the
queue, this is a good time to process it before busy-waiting on other
CPUs. On other architectures this call will quickly do nothing, as the
queue will be empty.

OK, I will bite...

How do we know that another entry will not get added by some other CPU
just before this new call to flush_smp_call_function_queue()?

							Thanx, Paul

quoted

Signed-off-by: Mark Tomlinson <redacted>
---
  kernel/smp.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index a0bb56bd8dda..3c4467654ab0 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c

@@ -884,6 +884,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
  		local_irq_restore(flags);
  	}
  
+	flush_smp_call_function_queue();
+
  	if (run_remote && wait) {
  		for_each_cpu(cpu, cfd->cpumask) {
  			call_single_data_t *csd;