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Subject[PATCH 09/11] job control: reorganize wait_task_stopped()
Date
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stopped condition might have been replaced by continued state by the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.

It seems that WNOHANG wait correctness has never been guaranteed and
everybody has been happy with it for very long time. As such,
although this reorganization improves the situation a bit, I don't
consider this to be a bug fix.

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

diff --git a/kernel/exit.c b/kernel/exit.c
index 5cbc83e..3383793 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -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
--
1.7.1


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