staging: line6: avoid __sync_fetch_and_{and,or}

__sync_fetch_and_and and __sync_fetch_and_or are functions that are provided
by gcc and depending on the target architecture may be implemented in libgcc,
which is not always available in the kernel. This leads to a build failure
on ARMv5:

drivers/built-in.o: In function `line6_pcm_release':
:(.text+0x3bfe80): undefined reference to `__sync_fetch_and_and_4'
drivers/built-in.o: In function `line6_pcm_acquire':
:(.text+0x3bff30): undefined reference to `__sync_fetch_and_or_4'

To work around this, we can use the kernel-provided cmpxchg macro.

Build-tested only.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Markus Grabner <grabner@icg.tugraz.at>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/line6/pcm.c

@@ -107,11 +107,15 @@ static bool test_flags(unsigned long flags0, unsigned long flags1,
 
 int line6_pcm_acquire(struct snd_line6_pcm *line6pcm, int channels)
 {
-	unsigned long flags_old =
-	    __sync_fetch_and_or(&line6pcm->flags, channels);
-	unsigned long flags_new = flags_old | channels;
-	unsigned long flags_final = flags_old;
-	int err = 0;
+	unsigned long flags_old, flags_new, flags_final;
+	int err;
+
+	do {
+		flags_old = ACCESS_ONCE(line6pcm->flags);
+		flags_new = flags_old | channels;
+	} while (cmpxchg(&line6pcm->flags, flags_old, flags_new) != flags_old);
+
+	flags_final = flags_old;
 
 	line6pcm->prev_fbuf = NULL;
 
@@ -197,9 +201,12 @@ int line6_pcm_acquire(struct snd_line6_pcm *line6pcm, int channels)
 
 int line6_pcm_release(struct snd_line6_pcm *line6pcm, int channels)
 {
-	unsigned long flags_old =
-	    __sync_fetch_and_and(&line6pcm->flags, ~channels);
-	unsigned long flags_new = flags_old & ~channels;
+	unsigned long flags_old, flags_new;
+
+	do {
+		flags_old = ACCESS_ONCE(line6pcm->flags);
+		flags_new = flags_old & ~channels;
+	} while (cmpxchg(&line6pcm->flags, flags_old, flags_new) != flags_old);
 
 	if (test_flags(flags_new, flags_old, LINE6_BITS_CAPTURE_STREAM))
 		line6_unlink_audio_in_urbs(line6pcm);