pstore: Make spinlock per zone instead of global

Currently pstore has a global spinlock for all zones. Since the zones
are independent and modify different areas of memory, there's no need
to have a global lock, so we should use a per-zone lock as introduced
here. Also, when ramoops's ftrace use-case has a FTRACE_PER_CPU flag
introduced later, which splits the ftrace memory area into a single zone
per CPU, it will eliminate the need for locking. In preparation for this,
make the locking optional.
Signed-off-by: Joel Fernandes <joelaf@google.com>
[kees: updated commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/ram_core.c

@@ -48,8 +48,6 @@ static inline size_t buffer_start(struct persistent_ram_zone *prz)
 	return atomic_read(&prz->buffer->start);
 }
 
-static DEFINE_RAW_SPINLOCK(buffer_lock);
-
 /* increase and wrap the start pointer, returning the old value */
 static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
 {
@@ -57,7 +55,7 @@ static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
 	int new;
 	unsigned long flags;
 
-	raw_spin_lock_irqsave(&buffer_lock, flags);
+	raw_spin_lock_irqsave(&prz->buffer_lock, flags);
 
 	old = atomic_read(&prz->buffer->start);
 	new = old + a;
@@ -65,7 +63,7 @@ static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
 		new -= prz->buffer_size;
 	atomic_set(&prz->buffer->start, new);
 
-	raw_spin_unlock_irqrestore(&buffer_lock, flags);
+	raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
 
 	return old;
 }
@@ -77,7 +75,7 @@ static void buffer_size_add(struct persistent_ram_zone *prz, size_t a)
 	size_t new;
 	unsigned long flags;
 
-	raw_spin_lock_irqsave(&buffer_lock, flags);
+	raw_spin_lock_irqsave(&prz->buffer_lock, flags);
 
 	old = atomic_read(&prz->buffer->size);
 	if (old == prz->buffer_size)
@@ -89,7 +87,7 @@ static void buffer_size_add(struct persistent_ram_zone *prz, size_t a)
 	atomic_set(&prz->buffer->size, new);
 
 exit:
-	raw_spin_unlock_irqrestore(&buffer_lock, flags);
+	raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
 }
 
 static void notrace persistent_ram_encode_rs8(struct persistent_ram_zone *prz,
@@ -493,6 +491,7 @@ static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 sig,
 
 	prz->buffer->sig = sig;
 	persistent_ram_zap(prz);
+	prz->buffer_lock = __RAW_SPIN_LOCK_UNLOCKED(buffer_lock);
 
 	return 0;
 }

include/linux/pstore_ram.h

@@ -40,6 +40,7 @@ struct persistent_ram_zone {
 	void *vaddr;
 	struct persistent_ram_buffer *buffer;
 	size_t buffer_size;
+	raw_spinlock_t buffer_lock;
 
 	/* ECC correction */
 	char *par_buffer;