hw-breakpoints: Modify breakpoints without unregistering them

Currently, when ptrace needs to modify a breakpoint, like disabling
it, changing its address, type or len, it calls
modify_user_hw_breakpoint(). This latter will perform the heavy and
racy task of unregistering the old breakpoint and registering a new
one.

This is racy as someone else might steal the reserved breakpoint
slot under us, which is undesired as the breakpoint is only
supposed to be modified, sometimes in the middle of a debugging
workflow. We don't want our slot to be stolen in the middle.

So instead of unregistering/registering the breakpoint, just
disable it while we modify its breakpoint fields and re-enable it
after if necessary.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prasad <prasad@linux.vnet.ibm.com>
LKML-Reference: <1260347148-5519-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/hw_breakpoint.h

@@ -55,7 +55,7 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 			    struct task_struct *tsk);
 
 /* FIXME: only change from the attr, and don't unregister */
-extern struct perf_event *
+extern int
 modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr);
 
 /*
@@ -91,7 +91,7 @@ static inline struct perf_event *
 register_user_hw_breakpoint(struct perf_event_attr *attr,
 			    perf_overflow_handler_t triggered,
 			    struct task_struct *tsk)	{ return NULL; }
-static inline struct perf_event *
+static inline int
 modify_user_hw_breakpoint(struct perf_event *bp,
 			  struct perf_event_attr *attr)	{ return NULL; }
 static inline struct perf_event *