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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  stopped condition might have been replaced by continued state by the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 <tj@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
This commit is contained in:
Tejun Heo 2011-05-12 10:47:23 +02:00 committed by Oleg Nesterov
parent 40ae717d1e
commit 19e274630c
1 changed files with 23 additions and 7 deletions
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30
kernel/exit.c
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@ -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