powerpc/pseries: Introduce rwlock to gatekeep DTLB usage

Since we would be introducing a new user of the DTL buffer in a
subsequent patch, we need a way to gatekeep use of the DTL buffer.

The current debugfs interface for DTL allows registering and opening
cpu-specific DTL buffers. Cpu specific files are exposed under
debugfs 'powerpc/dtl/' node, and changing 'dtl_event_mask' in the same
directory enables controlling the event mask used when registering DTL
buffer for a particular cpu.

Subsequently, we will be introducing a user of the DTL buffers that
registers access to the DTL buffers across all cpus with the same event
mask. To ensure these two users do not step on each other, we introduce
a rwlock to gatekeep DTL buffer access. This fits the requirement of the
current debugfs interface wanting to allow multiple independent
cpu-specific users (read lock), and the subsequent user wanting
exclusive access (write lock).

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 10 insertions, 1 deletions

diff --git a/arch/powerpc/platforms/pseries/dtl.c b/arch/powerpc/platforms/pseries/dtl.c
index fb05804adb2f..ae4fb2a68758 100644
--- a/arch/powerpc/platforms/pseries/dtl.c
+++ b/arch/powerpc/platforms/pseries/dtl.c
@@ -193,11 +193,16 @@ static int dtl_enable(struct dtl *dtl)
 	if (dtl->buf)
 		return -EBUSY;
 
+	/* ensure there are no other conflicting dtl users */
+	if (!read_trylock(&dtl_access_lock))
+		return -EBUSY;
+
 	n_entries = dtl_buf_entries;
 	buf = kmem_cache_alloc_node(dtl_cache, GFP_KERNEL, cpu_to_node(dtl->cpu));
 	if (!buf) {
 		printk(KERN_WARNING "%s: buffer alloc failed for cpu %d\n",
 				__func__, dtl->cpu);
+		read_unlock(&dtl_access_lock);
 		return -ENOMEM;
 	}
 
@@ -214,8 +219,11 @@ static int dtl_enable(struct dtl *dtl)
 	}
 	spin_unlock(&dtl->lock);
 
-	if (rc)
+	if (rc) {
+		read_unlock(&dtl_access_lock);
 		kmem_cache_free(dtl_cache, buf);
+	}
+
 	return rc;
 }
 
@@ -227,6 +235,7 @@ static void dtl_disable(struct dtl *dtl)
 	dtl->buf = NULL;
 	dtl->buf_entries = 0;
 	spin_unlock(&dtl->lock);
+	read_unlock(&dtl_access_lock);
 }
 
 /* file interface */