[PATCH 4/4] s390/maccess: Remove spinlock from __s390_kernel_write()

[Heiko Carstens]

The spinlock in __s390_kernel_write() was introduced to serialize the
read-modify-write sequence against concurrent callers. Now that all
relevant runtime callers of s390_kernel_write() hold the text_mutex, the
spinlock is not required anymore.

Replace it with preempt_disable() / preempt_enable() and sync_core().
Previously, CPU serialization was implicitly done by other
instructions. Make this explicit with sync_core() instead of
relying on that.

Preemption is disabled so that sync_core() issues the CPU serialization
instruction on the same CPU that executed the stores. This completes all
previous stores and makes them visible to other CPUs.

Other CPUs need to perform CPU serialization, e.g. with text_poke_sync(),
to discard already prefetched instructions.

Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
---
 arch/s390/mm/maccess.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/s390/mm/maccess.c b/arch/s390/mm/maccess.c
index f39968dd8063..cda3f81dfbc2 100644
--- a/arch/s390/mm/maccess.c
+++ b/arch/s390/mm/maccess.c
@@ -6,6 +6,7 @@
  *
  */
 
+#include <linux/text-patching.h>
 #include <linux/uaccess.h>
 #include <linux/kernel.h>
 #include <linux/types.h>
@@ -60,22 +61,20 @@ static notrace long s390_kernel_write_odd(void *dst, const void *src, size_t siz
  * bytes from destination at an eight byte boundary, modifies the bytes
  * requested and writes the result back in a loop.
  */
-static DEFINE_SPINLOCK(s390_kernel_write_lock);
-
 notrace void *__s390_kernel_write(void *dst, const void *src, size_t size)
 {
 	void *tmp = dst;
-	unsigned long flags;
 	long copied;
 
-	spin_lock_irqsave(&s390_kernel_write_lock, flags);
+	preempt_disable();
 	while (size) {
 		copied = s390_kernel_write_odd(tmp, src, size);
 		tmp += copied;
 		src += copied;
 		size -= copied;
 	}
-	spin_unlock_irqrestore(&s390_kernel_write_lock, flags);
+	sync_core();
+	preempt_enable();
 
 	return dst;
 }
-- 
2.53.0