linux-kernel - [PATCH v12 11/13] um: nommu: disable SMP on nommu UML

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Message-ID: <54839396f81bc2755728a53912bd8fcb19b889a1.1762075876.git.thehajime@gmail.com>
Date: Sun,  2 Nov 2025 18:49:36 +0900
From: Hajime Tazaki <thehajime@...il.com>
To: linux-um@...ts.infradead.org
Cc: thehajime@...il.com,
	ricarkol@...gle.com,
	Liam.Howlett@...cle.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH v12 11/13] um: nommu: disable SMP on nommu UML

CONFIG_SMP doesn't work with nommu UML since fs register handling of
host does conflict with thread local storage (more specifically,
the variable signals_enabled).

Thus this commit disables the CONFIG option and the TLS variables.

Signed-off-by: Hajime Tazaki <thehajime@...il.com>
---
 arch/um/os-Linux/internal.h | 8 ++++++++
 arch/x86/um/Kconfig         | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/um/os-Linux/internal.h b/arch/um/os-Linux/internal.h
index bac9fcc8c14c..25cb5cc931c1 100644
--- a/arch/um/os-Linux/internal.h
+++ b/arch/um/os-Linux/internal.h
@@ -6,6 +6,14 @@
 #include <stub-data.h>
 #include <signal.h>
 
+/* NOMMU doesn't work with thread-local storage used in CONFIG_SMP,
+ * due to the dependency on host_fs variable switch upon user/kernel
+ * context so, disable TLS until NOMMU supports SMP.
+ */
+#ifndef CONFIG_MMU
+#define __thread
+#endif
+
 /*
  * elf_aux.c
  */
diff --git a/arch/x86/um/Kconfig b/arch/x86/um/Kconfig
index c52fb5cb8d21..2bc18ecad783 100644
--- a/arch/x86/um/Kconfig
+++ b/arch/x86/um/Kconfig
@@ -13,7 +13,7 @@ config UML_X86
 	select ARCH_USE_QUEUED_SPINLOCKS
 	select DCACHE_WORD_ACCESS
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
-	select UML_SUBARCH_SUPPORTS_SMP if X86_CX8
+	select UML_SUBARCH_SUPPORTS_SMP if X86_CX8 && MMU
 
 config 64BIT
 	bool "64-bit kernel" if "$(SUBARCH)" = "x86"
-- 
2.43.0