[PATCH] alpha: Fix SMP shutdown hang due to missing memory barriers

Alpha has a very weak memory model. halt() makes no guarantee that
pending stores have drained from the store buffer. If set_cpu_present()
stores are still buffered when a secondary CPU halts, they are lost,
and the boot CPU spins forever in the cpu_present_mask wait loop.

Add mb() before halt() on secondary CPUs to flush the store buffer,
and use smp_mb() in the boot CPU's poll loop instead of the
compiler-only barrier() to ensure it observes secondary CPUs' stores.

This avoids a deadlock on shutdown on EV7/Marvel platforms.

Cc: [email protected]
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Matt Turner <[email protected]>
---
 arch/alpha/kernel/process.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git ./arch/alpha/kernel/process.c ./arch/alpha/kernel/process.c
index 06522451f018..d50f9cfd8333 100644
--- ./arch/alpha/kernel/process.c
+++ ./arch/alpha/kernel/process.c
@@ -99,6 +99,7 @@ common_shutdown_1(void *generic_ptr)
 		*pflags = flags;
 		set_cpu_present(cpuid, false);
 		set_cpu_possible(cpuid, false);
+		mb();
 		halt();
 	}
 #endif
@@ -127,7 +128,7 @@ common_shutdown_1(void *generic_ptr)
 	set_cpu_present(boot_cpuid, false);
 	set_cpu_possible(boot_cpuid, false);
 	while (!cpumask_empty(cpu_present_mask))
-		barrier();
+		smp_mb();
 #endif
 
 	/* If booted from SRM, reset some of the original environment. */
-- 
2.53.0