[PATCH] smp_rmb_cond

[Matthew Wilcox <willy@infradead.org>]

i see worse inlining decisions from gcc with this.  maybe you see
an improvement that would justify it?

[ref: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99998]

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 4819d5e5a335..4cbc5bd5bcdd 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -60,6 +60,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 #define __smp_mb()	asm volatile("lock; addl $0,-4(%%rsp)" ::: "memory", "cc")
 #endif
 #define __smp_rmb()	dma_rmb()
+#define smp_rmb_cond(x)	barrier()
 #define __smp_wmb()	barrier()
 #define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
 
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 640f09479bdf..cc0c864f90dc 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -89,6 +89,10 @@
 
 #endif	/* CONFIG_SMP */
 
+#ifndef smp_rmb_cond
+#define smp_rmb_cond(x) do { if (x) smp_rmb(); } while (0)
+#endif
+
 #ifndef __smp_store_mb
 #define __smp_store_mb(var, value)  do { WRITE_ONCE(var, value); __smp_mb(); } while (0)
 #endif
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c08e0a6..c45d491e9245 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -522,8 +522,7 @@ static inline int PageUptodate(struct page *page)
 	 *
 	 * See SetPageUptodate() for the other side of the story.
 	 */
-	if (ret)
-		smp_rmb();
+	smp_rmb_cond(ret);
 
 	return ret;
 }
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 7a1414622051..260ef2474ff2 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -89,8 +89,7 @@ static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
 	 * Make sure that all old data have been read before the buffer
 	 * was reset. This is not needed when we just append data.
 	 */
-	if (!len)
-		smp_rmb();
+	smp_rmb_cond(!len);
 
 	va_copy(ap, args);
 	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);