job control: reorganize wait_task_stopped()

PATCH New Tejun Heo 1 participant 1 message May 12, 2011

Share & tools

Tejun Heo

author

2011-05-12 08:47 UTC #0 lore raw

PATCH 1/1 job control: reorganize wait_task_stopped()

wait_task_stopped() tested task_stopped_code() without acquiring siglock and, if stop condition existed, called wait_task_stopped() and directly returned the result. This patch moves the initial task_stopped_code() testing into wait_task_stopped() and make wait_consider_task() fall through to wait_task_continue() on 0 return.

This is for the following two reasons.

* Because the initial task_stopped_code() test is done without acquiring siglock, it may race against SIGCONT generation. The stopped condition might have been replaced by continued state by the time wait_task_stopped() acquired siglock. This may lead to unexpected failure of WNOHANG waits.

This reorganization addresses this single race case but there are other cases - TASK_RUNNING -> TASK_STOPPED transition and EXIT_* transitions.

* Scheduled ptrace updates require changes to the initial test which would fit better inside wait_task_stopped().

Signed-off-by Tejun Heo <[email protected]>

Reviewed-by Oleg Nesterov <[email protected]>

---
Dropped the "it's not a bug fix" paragraph from description and added Reviewed-by.

Thanks.

 kernel/exit.c |   30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

kernel/exit.c +23 -7

Index: work/kernel/exit.c
===================================================================
@@ -1377,11 +1377,23 @@ static int *task_stopped_code(struct tas
 	return NULL;
 }
 
-/*
- * Handle sys_wait4 work for one task in state TASK_STOPPED.  We hold
- * read_lock(&tasklist_lock) on entry.  If we return zero, we still hold
- * the lock and this task is uninteresting.  If we return nonzero, we have
- * released the lock and the system call should return.
+/**
+ * wait_task_stopped - Wait for %TASK_STOPPED or %TASK_TRACED
+ * @wo: wait options
+ * @ptrace: is the wait for ptrace
+ * @p: task to wait for
+ *
+ * Handle sys_wait4() work for %p in state %TASK_STOPPED or %TASK_TRACED.
+ *
+ * CONTEXT:
+ * read_lock(&tasklist_lock), which is released if return value is
+ * non-zero.  Also, grabs and releases @p->sighand->siglock.
+ *
+ * RETURNS:
+ * 0 if wait condition didn't exist and search for other wait conditions
+ * should continue.  Non-zero return, -errno on failure and @p's pid on
+ * success, implies that tasklist_lock is released and wait condition
+ * search should terminate.
  */
 static int wait_task_stopped(struct wait_opts *wo,
 				int ptrace, struct task_struct *p)

@@ -1397,6 +1409,9 @@ static int wait_task_stopped(struct wait
 	if (!ptrace && !(wo->wo_flags & WUNTRACED))
 		return 0;
 
+	if (!task_stopped_code(p, ptrace))
+		return 0;
+
 	exit_code = 0;
 	spin_lock_irq(&p->sighand->siglock);

@@ -1607,8 +1622,9 @@ static int wait_consider_task(struct wai
 	 * Wait for stopped.  Depending on @ptrace, different stopped state
 	 * is used and the two don't interact with each other.
 	 */
-	if (task_stopped_code(p, ptrace))
-		return wait_task_stopped(wo, ptrace, p);
+	ret = wait_task_stopped(wo, ptrace, p);
+	if (ret)
+		return ret;
 
 	/*
 	 * Wait for continued.  There's only one continued state and the