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Message-ID: <20240626151131.550535-2-torvalds@linux-foundation.org>
Date: Wed, 26 Jun 2024 08:11:32 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Uros Bizjak <ubizjak@...il.com>,
Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org,
x86@...nel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
kernel test robot <lkp@...el.com>
Subject: [PATCH v2] x86-32: fix cmpxchg8b_emu build error with clang
The kernel test robot reported that clang no longer compiles the 32-bit
x86 kernel in some configurations due to commit 95ece48165c1
("locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}()
functions").
The build fails with
arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly requires more registers than available
and the reason seems to be that not only does the cmpxchg8b instruction
need four fixed registers (EDX:EAX and ECX:EBX), with the emulation
fallback the inline asm also wants a fifth fixed register for the
address (it uses %esi for that, but that's just a software convention
with cmpxchg8b_emu).
Avoiding using another pointer input to the asm (and just forcing it to
use the "0(%esi)" addressing that we end up requiring for the sw
fallback) seems to fix the issue.
Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406230912.F6XFIyA6-lkp@intel.com/
Fixes: 95ece48165c1 ("locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}() functions")
Link: https://lore.kernel.org/all/202406230912.F6XFIyA6-lkp@intel.com/
Suggested-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
---
This is the minimal patch, with the other simplifications removed.
I still think that the case where somebody passes in a "oldp" that can
change during the cmpxchg is entirely broken, and is not actually valid.
If it was valid, we'd have to use READ_ONCE() and WRITE_ONCE() in the
"oldp" updates anyway just to make verification tools happy. We don't.
But hey, that simplification doesn't matter for fixing this build issue,
so let's keep the change minimal.
arch/x86/include/asm/cmpxchg_32.h | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
index ed2797f132ce..62cef2113ca7 100644
--- a/arch/x86/include/asm/cmpxchg_32.h
+++ b/arch/x86/include/asm/cmpxchg_32.h
@@ -93,10 +93,9 @@ static __always_inline bool __try_cmpxchg64_local(volatile u64 *ptr, u64 *oldp,
\
asm volatile(ALTERNATIVE(_lock_loc \
"call cmpxchg8b_emu", \
- _lock "cmpxchg8b %[ptr]", X86_FEATURE_CX8) \
- : [ptr] "+m" (*(_ptr)), \
- "+a" (o.low), "+d" (o.high) \
- : "b" (n.low), "c" (n.high), "S" (_ptr) \
+ _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
+ : "+a" (o.low), "+d" (o.high) \
+ : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
: "memory"); \
\
o.full; \
@@ -122,12 +121,11 @@ static __always_inline u64 arch_cmpxchg64_local(volatile u64 *ptr, u64 old, u64
\
asm volatile(ALTERNATIVE(_lock_loc \
"call cmpxchg8b_emu", \
- _lock "cmpxchg8b %[ptr]", X86_FEATURE_CX8) \
+ _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
CC_SET(e) \
: CC_OUT(e) (ret), \
- [ptr] "+m" (*(_ptr)), \
"+a" (o.low), "+d" (o.high) \
- : "b" (n.low), "c" (n.high), "S" (_ptr) \
+ : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
: "memory"); \
\
if (unlikely(!ret)) \
--
2.45.1.209.gc6f12300df
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