wait_consider_task() checks EXIT_ZOMBIE after EXIT_DEAD/EXIT_TRACE and both checks can fail if we race with EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE change in between, gcc needs to reload p->exit_state after security_task_wait(). In this case ->notask_error will be wrongly cleared and do_wait() can hang forever if it was the last eligible child.
Many thanks to Arne who carefully investigated the problem. Note: this bug is very old but it was pure theoretical until b3ab03160dfa "wait: completely ignore the EXIT_DEAD tasks". Before this commit "-O2" was probably enough to guarantee that compiler won't read ->exit_state twice. Signed-off-by: Oleg Nesterov <[email protected]> Reported-by: Arne Goedeke <[email protected]> Tested-by: Arne Goedeke <[email protected]> Cc: <[email protected]> # v3.15+ --- kernel/exit.c | 12 +++++++++--- 1 files changed, 9 insertions(+), 3 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 43394f7..37efc8e 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1303,9 +1303,15 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p) static int wait_consider_task(struct wait_opts *wo, int ptrace, struct task_struct *p) { + /* + * We can race with wait_task_zombie() from another thread. + * Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition + * can't confuse the checks below. + */ + int exit_state = ACCESS_ONCE(p->exit_state); int ret; - if (unlikely(p->exit_state == EXIT_DEAD)) + if (unlikely(exit_state == EXIT_DEAD)) return 0; ret = eligible_child(wo, p); @@ -1326,7 +1332,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, return 0; } - if (unlikely(p->exit_state == EXIT_TRACE)) { + if (unlikely(exit_state == EXIT_TRACE)) { /* * ptrace == 0 means we are the natural parent. In this case * we should clear notask_error, debugger will notify us. @@ -1353,7 +1359,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, } /* slay zombie? */ - if (p->exit_state == EXIT_ZOMBIE) { + if (exit_state == EXIT_ZOMBIE) { /* we don't reap group leaders with subthreads */ if (!delay_group_leader(p)) { /* -- 1.5.5.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
