[v2,13/13] arm64/fpsimd: signal: Simplify preserve_tpidr2_context()

Commit Message

Mark Rutland April 9, 2025, 4:40 p.m. UTC

During a context-switch, tls_thread_switch() reads and writes a task's
thread_struct::tpidr2_el0 field. Other code shouldn't access this field
for an active task, as such accesses would form a data-race with a
concurrent context-switch.

The usage in preserve_tpidr2_context() is suspicious, but benign as any
race with a context switch will write the same value back to
current->thread.tpidr2_el0.

Make this clearer and match restore_tpidr2_context() by using a
temporary variable instead, avoiding the (benign) data-race.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Will Deacon <[e-mail]>
Reviewed-by: Mark Brown <[e-mail]>
---
 arch/arm64/kernel/signal.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

Patch

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 0de9c452c6c0e..73f1ab56d81b2 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -478,13 +478,12 @@  extern int preserve_sve_context(void __user *ctx);
 
 static int preserve_tpidr2_context(struct tpidr2_context __user *ctx)
 {
+	u64 tpidr2_el0 = read_sysreg_s(SYS_TPIDR2_EL0);
 	int err = 0;
 
-	current->thread.tpidr2_el0 = read_sysreg_s(SYS_TPIDR2_EL0);
-
 	__put_user_error(TPIDR2_MAGIC, &ctx->head.magic, err);
 	__put_user_error(sizeof(*ctx), &ctx->head.size, err);
-	__put_user_error(current->thread.tpidr2_el0, &ctx->tpidr2, err);
+	__put_user_error(tpidr2_el0, &ctx->tpidr2, err);
 
 	return err;
 }