ptrace: revert "Prepare to fix racy accesses on task breakpoints"

This reverts commit bf26c01849 ("Prepare to fix racy accesses on task
breakpoints").

The patch was fine but we can no longer race with SIGKILL after commit
9899d11f65 ("ptrace: ensure arch_ptrace/ptrace_request can never race
with SIGKILL"), the __TASK_TRACED tracee can't be woken up and
->ptrace_bps[] can't go away.

Now that ptrace_get_breakpoints/ptrace_put_breakpoints have no callers,
we can kill them and remove task->ptrace_bp_refcnt.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Michael Neuling <mikey@neuling.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ptrace.h

@@ -142,9 +142,6 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
 {
 	INIT_LIST_HEAD(&child->ptrace_entry);
 	INIT_LIST_HEAD(&child->ptraced);
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-	atomic_set(&child->ptrace_bp_refcnt, 1);
-#endif
 	child->jobctl = 0;
 	child->ptrace = 0;
 	child->parent = child->real_parent;
@@ -351,11 +348,4 @@ extern int task_current_syscall(struct task_struct *target, long *callno,
 				unsigned long args[6], unsigned int maxargs,
 				unsigned long *sp, unsigned long *pc);
 
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-extern int ptrace_get_breakpoints(struct task_struct *tsk);
-extern void ptrace_put_breakpoints(struct task_struct *tsk);
-#else
-static inline void ptrace_put_breakpoints(struct task_struct *tsk) { }
-#endif /* CONFIG_HAVE_HW_BREAKPOINT */
-
 #endif

include/linux/sched.h

@@ -1401,9 +1401,6 @@ struct task_struct {
 	} memcg_batch;
 	unsigned int memcg_kmem_skip_account;
 #endif
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-	atomic_t ptrace_bp_refcnt;
-#endif
 #ifdef CONFIG_UPROBES
 	struct uprobe_task *utask;
 #endif

kernel/exit.c

@@ -808,7 +808,7 @@ void do_exit(long code)
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
 	 */
-	ptrace_put_breakpoints(tsk);
+	flush_ptrace_hw_breakpoint(tsk);
 
 	exit_notify(tsk, group_dead);
 #ifdef CONFIG_NUMA

kernel/ptrace.c

@@ -1221,19 +1221,3 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
 	return ret;
 }
 #endif	/* CONFIG_COMPAT */
-
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-int ptrace_get_breakpoints(struct task_struct *tsk)
-{
-	if (atomic_inc_not_zero(&tsk->ptrace_bp_refcnt))
-		return 0;
-
-	return -1;
-}
-
-void ptrace_put_breakpoints(struct task_struct *tsk)
-{
-	if (atomic_dec_and_test(&tsk->ptrace_bp_refcnt))
-		flush_ptrace_hw_breakpoint(tsk);
-}
-#endif /* CONFIG_HAVE_HW_BREAKPOINT */