Commit 19e27463 authored by Tejun Heo's avatar Tejun Heo Committed by Oleg Nesterov
Browse files

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  stopped condition might have been replaced by continued state by the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: default avatarTejun Heo <tj@kernel.org>
Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
parent 40ae717d

kernel/exit.c

+23 −7
Original line number Diff line number Diff line
@@ -1377,11 +1377,23 @@ static int *task_stopped_code(struct task_struct *p, bool ptrace) 
	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_opts *wo, 
	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 wait_opts *wo, int ptrace, 
	 * 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

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