From 536fa402221f09633e7c5801b327055ab716a363 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 5 Sep 2014 11:14:48 -0700 Subject: compiler: Allow 1- and 2-byte smp_load_acquire() and smp_store_release() CPUs without single-byte and double-byte loads and stores place some "interesting" requirements on concurrent code. For example (adapted from Peter Hurley's test code), suppose we have the following structure: struct foo { spinlock_t lock1; spinlock_t lock2; char a; /* Protected by lock1. */ char b; /* Protected by lock2. */ }; struct foo *foop; Of course, it is common (and good) practice to place data protected by different locks in separate cache lines. However, if the locks are rarely acquired (for example, only in rare error cases), and there are a great many instances of the data structure, then memory footprint can trump false-sharing concerns, so that it can be better to place them in the same cache cache line as above. But if the CPU does not support single-byte loads and stores, a store to foop->a will do a non-atomic read-modify-write operation on foop->b, which will come as a nasty surprise to someone holding foop->lock2. So we now require CPUs to support single-byte and double-byte loads and stores. Therefore, this commit adjusts the definition of __native_word() to allow these sizes to be used by smp_load_acquire() and smp_store_release(). Signed-off-by: Paul E. McKenney Cc: Peter Zijlstra --- include/linux/compiler.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/compiler.h') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index a1c81f80978e..49811cdddaa5 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -385,7 +385,7 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int /* Is this type a native word size -- useful for atomic operations */ #ifndef __native_word -# define __native_word(t) (sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long)) +# define __native_word(t) (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long)) #endif /* Compile time object size, -1 for unknown */ -- cgit v1.2.3 From 927609d622a3773995f84bc03b4564f873cf0e22 Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Tue, 25 Nov 2014 10:16:39 +0100 Subject: kernel: tighten rules for ACCESS ONCE Now that all non-scalar users of ACCESS_ONCE have been converted to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only work on scalar types. This variant was proposed by Alexei Starovoitov. Signed-off-by: Christian Borntraeger Reviewed-by: Paul E. McKenney --- include/linux/compiler.h | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) (limited to 'include/linux/compiler.h') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index a1c81f80978e..5e186bf6f6c1 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -447,12 +447,23 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int * to make the compiler aware of ordering is to put the two invocations of * ACCESS_ONCE() in different C statements. * - * This macro does absolutely -nothing- to prevent the CPU from reordering, - * merging, or refetching absolutely anything at any time. Its main intended - * use is to mediate communication between process-level code and irq/NMI - * handlers, all running on the same CPU. + * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE + * on a union member will work as long as the size of the member matches the + * size of the union and the size is smaller than word size. + * + * The major use cases of ACCESS_ONCE used to be (1) Mediating communication + * between process-level code and irq/NMI handlers, all running on the same CPU, + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise + * mutilate accesses that either do not require ordering or that interact + * with an explicit memory barrier or atomic instruction that provides the + * required ordering. + * + * If possible use READ_ONCE/ASSIGN_ONCE instead. */ -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define __ACCESS_ONCE(x) ({ \ + __maybe_unused typeof(x) __var = 0; \ + (volatile typeof(x) *)&(x); }) +#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */ #ifdef CONFIG_KPROBES -- cgit v1.2.3 From c5b19946eb76c67566aae6a84bf2b10ad59295ea Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Mon, 12 Jan 2015 12:13:39 +0100 Subject: kernel: Fix sparse warning for ACCESS_ONCE Commit 927609d622a3 ("kernel: tighten rules for ACCESS ONCE") results in sparse warnings like "Using plain integer as NULL pointer" - Let's add a type cast to the dummy assignment. To avoid warnings lik "sparse: warning: cast to restricted __hc32" we also use __force on that cast. Fixes: 927609d622a3 ("kernel: tighten rules for ACCESS ONCE") Signed-off-by: Christian Borntraeger --- include/linux/compiler.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/compiler.h') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 5e186bf6f6c1..7bebf0563d18 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -461,7 +461,7 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int * If possible use READ_ONCE/ASSIGN_ONCE instead. */ #define __ACCESS_ONCE(x) ({ \ - __maybe_unused typeof(x) __var = 0; \ + __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \ (volatile typeof(x) *)&(x); }) #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) -- cgit v1.2.3 From 61f552141c9c0e88b3fdc7046265781ffd8fa68a Mon Sep 17 00:00:00 2001 From: Heiko Carstens Date: Sun, 18 Jan 2015 16:45:42 +0100 Subject: ftrace: let notrace function attribute disable hotpatching if necessary gcc supports an s390 specific function attribute called "hotpatch". It can be used to specify the number of halfwords that shall be added before and after a function and which shall be filled with nops for runtime patching. s390 will use the hotpatch attribute for function tracing, therefore make sure that the notrace function attribute either disables the mcount call or in case of hotpatch nop generation. Acked-by: Steven Rostedt Signed-off-by: Heiko Carstens Signed-off-by: Martin Schwidefsky --- include/linux/compiler.h | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'include/linux/compiler.h') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d5ad7b1118fc..1ef679f4b88e 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -54,7 +54,11 @@ extern void __chk_io_ptr(const volatile void __iomem *); #include #endif +#ifdef CC_USING_HOTPATCH +#define notrace __attribute__((hotpatch(0,0))) +#else #define notrace __attribute__((no_instrument_function)) +#endif /* Intel compiler defines __GNUC__. So we will overwrite implementations * coming from above header files here -- cgit v1.2.3 From dd36929720f40f17685e841ae0d4c581c165ea60 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 20 Feb 2015 15:46:31 -0800 Subject: kernel: make READ_ONCE() valid on const arguments The use of READ_ONCE() causes lots of warnings witht he pending paravirt spinlock fixes, because those ends up having passing a member to a 'const' structure to READ_ONCE(). There should certainly be nothing wrong with using READ_ONCE() with a const source, but the helper function __read_once_size() would cause warnings because it would drop the 'const' qualifier, but also because the destination would be marked 'const' too due to the use of 'typeof'. Use a union of types in READ_ONCE() to avoid this issue. Also make sure to use parenthesis around the macro arguments to avoid possible operator precedence issues. Tested-by: Ingo Molnar Cc: Christian Borntraeger Signed-off-by: Linus Torvalds --- include/linux/compiler.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) (limited to 'include/linux/compiler.h') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d1ec10a940ff..1b45e4a0519b 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -202,7 +202,7 @@ static __always_inline void data_access_exceeds_word_size(void) { } -static __always_inline void __read_once_size(volatile void *p, void *res, int size) +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; @@ -259,10 +259,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s */ #define READ_ONCE(x) \ - ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; }) + ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) #define WRITE_ONCE(x, val) \ - ({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; }) + ({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; }) #endif /* __KERNEL__ */ -- cgit v1.2.3