[RFC V3] test bit before clear files_struct bits

[Wang, Yalin]

Test bit before clear close_on_exec and open_fds,
by trace __clear_bit(), these 2 place are false in most times,
we test it so that we don't need clear_bit, and we can win
in most time.
Add *_if_need bitop non-atomic version.

Signed-off-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx>
---
 fs/file.c                               |  9 +++++++--
 include/asm-generic/bitops/non-atomic.h | 35 +++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index ee738ea..2e08e6f 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -209,7 +209,7 @@ static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
 
 static inline void __clear_close_on_exec(int fd, struct fdtable *fdt)
 {
-	__clear_bit(fd, fdt->close_on_exec);
+	__clear_bit_if_need(fd, fdt->close_on_exec);
 }
 
 static inline void __set_open_fd(int fd, struct fdtable *fdt)
@@ -222,6 +222,11 @@ static inline void __clear_open_fd(int fd, struct fdtable *fdt)
 	__clear_bit(fd, fdt->open_fds);
 }
 
+static inline void __clear_open_fd_if_need(int fd, struct fdtable *fdt)
+{
+	__clear_bit_if_need(fd, fdt->open_fds);
+}
+
 static int count_open_files(struct fdtable *fdt)
 {
 	int size = fdt->max_fds;
@@ -316,7 +321,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 			 * is partway through open().  So make sure that this
 			 * fd is available to the new process.
 			 */
-			__clear_open_fd(open_files - i, new_fdt);
+			__clear_open_fd_if_need(open_files - i, new_fdt);
 		}
 		rcu_assign_pointer(*new_fds++, f);
 	}
diff --git a/include/asm-generic/bitops/non-atomic.h b/include/asm-generic/bitops/non-atomic.h
index 697cc2b..0713e3b 100644
--- a/include/asm-generic/bitops/non-atomic.h
+++ b/include/asm-generic/bitops/non-atomic.h
@@ -105,4 +105,39 @@ static inline int test_bit(int nr, const volatile unsigned long *addr)
 	return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
 }
 
+/*
+ * __*_if_need version is used in cases that you don't want write a bit which
+ * have been set / clear, to avoid dirty cacheline.
+ */
+static inline void __set_bit_if_need(int nr, volatile unsigned long *addr)
+{
+	if (!test_bit(nr, addr))
+		__set_bit(nr, addr);
+}
+
+static inline void __clear_bit_if_need(int nr, volatile unsigned long *addr)
+{
+	if (test_bit(nr, addr))
+		__clear_bit(nr, addr);
+}
+
+static inline int __test_and_set_bit_if_need(int nr, volatile unsigned long *addr)
+{
+	if (!test_bit(nr, addr)) {
+		__set_bit(nr, addr);
+		return false;
+	} else {
+		return true;
+	}
+}
+
+static inline int __test_and_clear_bit_if_need(int nr, volatile unsigned long *addr)
+{
+	if (test_bit(nr, addr)) {
+		__clear_bit(nr, addr);
+		return true;
+	} else {
+		return false;
+	}
+}
 #endif /* _ASM_GENERIC_BITOPS_NON_ATOMIC_H_ */
-- 
2.2.2
--
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