hw_breakpoint: Add modify_bp_slot() function

Add the modify_bp_slot() function to keep slot numbers
correct when changing the breakpoint type.

Using existing __release_bp_slot()/__reserve_bp_slot()
call sequence to update the slot counts.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Milind Chabbi <chabbi.milind@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oleg Nesterov <onestero@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/20180312134548.31532-4-jolsa@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/hw_breakpoint.c

@@ -44,6 +44,7 @@
 #include <linux/list.h>
 #include <linux/cpu.h>
 #include <linux/smp.h>
+#include <linux/bug.h>
 
 #include <linux/hw_breakpoint.h>
 /*
@@ -344,6 +345,38 @@ void release_bp_slot(struct perf_event *bp)
 	mutex_unlock(&nr_bp_mutex);
 }
 
+static int __modify_bp_slot(struct perf_event *bp, u64 old_type)
+{
+	int err;
+
+	__release_bp_slot(bp, old_type);
+
+	err = __reserve_bp_slot(bp, bp->attr.bp_type);
+	if (err) {
+		/*
+		 * Reserve the old_type slot back in case
+		 * there's no space for the new type.
+		 *
+		 * This must succeed, because we just released
+		 * the old_type slot in the __release_bp_slot
+		 * call above. If not, something is broken.
+		 */
+		WARN_ON(__reserve_bp_slot(bp, old_type));
+	}
+
+	return err;
+}
+
+static int modify_bp_slot(struct perf_event *bp, u64 old_type)
+{
+	int ret;
+
+	mutex_lock(&nr_bp_mutex);
+	ret = __modify_bp_slot(bp, old_type);
+	mutex_unlock(&nr_bp_mutex);
+	return ret;
+}
+
 /*
  * Allow the kernel debugger to reserve breakpoint slots without
  * taking a lock using the dbg_* variant of for the reserve and
@@ -435,6 +468,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 	u64 old_addr = bp->attr.bp_addr;
 	u64 old_len = bp->attr.bp_len;
 	int old_type = bp->attr.bp_type;
+	bool modify = attr->bp_type != old_type;
 	int err = 0;
 
 	/*
@@ -452,12 +486,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 	bp->attr.bp_type = attr->bp_type;
 	bp->attr.bp_len = attr->bp_len;
 
-	if (attr->disabled)
-		goto end;
-
 	err = validate_hw_breakpoint(bp);
-	if (!err)
-		perf_event_enable(bp);
+	if (!err && modify)
+		err = modify_bp_slot(bp, old_type);
 
 	if (err) {
 		bp->attr.bp_addr = old_addr;
@@ -469,9 +500,10 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 		return err;
 	}
 
-end:
-	bp->attr.disabled = attr->disabled;
+	if (!attr->disabled)
+		perf_event_enable(bp);
 
+	bp->attr.disabled = attr->disabled;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(modify_user_hw_breakpoint);