linux-kernel - [PATCH 3.10 280/319] locking: Remove atomicy checks from {READ,WRITE}

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Date:   Sun,  5 Feb 2017 20:21:42 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 280/319] locking: Remove atomicy checks from {READ,WRITE}_ONCE

From: Peter Zijlstra <peterz@...radead.org>

commit 7bd3e239d6c6d1cad276e8f130b386df4234dcd7 upstream.

The fact that volatile allows for atomic load/stores is a special case
not a requirement for {READ,WRITE}_ONCE(). Their primary purpose is to
force the compiler to emit load/stores _once_.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Christian Borntraeger <borntraeger@...ibm.com>
Acked-by: Will Deacon <will.deacon@....com>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[wt: backported only for next patch]

Signed-off-by: Willy Tarreau <w@....eu>
---
 include/linux/compiler.h | 16 ----------------
 1 file changed, 16 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c6d06e9..53563d8 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -181,29 +181,16 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 
 #include <uapi/linux/types.h>
 
-static __always_inline void data_access_exceeds_word_size(void)
-#ifdef __compiletime_warning
-__compiletime_warning("data access exceeds word size and won't be atomic")
-#endif
-;
-
-static __always_inline void data_access_exceeds_word_size(void)
-{
-}
-
 static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
 {
 	switch (size) {
 	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
 	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
 	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
-#ifdef CONFIG_64BIT
 	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)res, (const void *)p, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }
@@ -214,13 +201,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
 	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
 	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
-#ifdef CONFIG_64BIT
 	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)p, (const void *)res, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }
-- 
2.8.0.rc2.1.gbe9624a