linux-kernel - [PATCH] powerpc: Select HAVE_FUTEX

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Message-Id: <20200919032009.8346-1-samuel@sholland.org>
Date:   Fri, 18 Sep 2020 22:20:09 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     linux-kernel@...r.kernel.org, Samuel Holland <samuel@...lland.org>
Subject: [PATCH] powerpc: Select HAVE_FUTEX_CMPXCHG

On powerpc, access_ok() succeeds for the NULL pointer. This breaks the
dynamic check in futex_detect_cmpxchg(), which expects -EFAULT. As a
result, robust futex operations are not functional on powerpc.

Since the architecture's futex_atomic_cmpxchg_inatomic() implementation
requires no runtime feature detection, we can select HAVE_FUTEX_CMPXCHG
to skip futex_detect_cmpxchg() and enable the use of robust futexes.

Signed-off-by: Samuel Holland <samuel@...lland.org>
---
 arch/powerpc/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index ad620637cbd1..5ad1deb0c669 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -196,6 +196,7 @@ config PPC
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
+	select HAVE_FUTEX_CMPXCHG
 	select HAVE_GCC_PLUGINS			if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC
 	select HAVE_HW_BREAKPOINT		if PERF_EVENTS && (PPC_BOOK3S || PPC_8xx)
 	select HAVE_IDE
-- 
2.26.2