signal: do_sigtimedwait() needs retarget_shared_pending()

do_sigtimedwait() changes current->blocked and thus it needs
set_current_blocked()->retarget_shared_pending().

We could use set_current_blocked() directly. It is fine to change
->real_blocked from all-zeroes to ->blocked and vice versa lockless,
but this is not immediately clear, looks racy, and needs a huge
comment to explain why this is correct.

To keep the things simple this patch adds the new static helper,
__set_task_blocked() which should be called with ->siglock held. This
way we can change both ->real_blocked and ->blocked atomically under
->siglock as the current code does. This is more understandable.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Matt Fleming <matt.fleming@linux.intel.com>

1 files changed, 17 insertions, 12 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 1ab89f677424..4d97e11d7672 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2299,6 +2299,18 @@ long do_no_restart_syscall(struct restart_block *param)
         return -EINTR;
 }
 
+static void __set_task_blocked(struct task_struct *tsk, const sigset_t *newset)
+{
+        if (signal_pending(tsk) && !thread_group_empty(tsk)) {
+                sigset_t newblocked;
+                /* A set of now blocked but previously unblocked signals. */
+                signandsets(&newblocked, newset, &current->blocked);
+                retarget_shared_pending(tsk, &newblocked);
+        }
+        tsk->blocked = *newset;
+        recalc_sigpending();
+}
+
 /**
  * set_current_blocked - change current->blocked mask
  * @newset: new mask
@@ -2311,14 +2323,7 @@ void set_current_blocked(const sigset_t *newset)
         struct task_struct *tsk = current;
 
         spin_lock_irq(&tsk->sighand->siglock);
-        if (signal_pending(tsk) && !thread_group_empty(tsk)) {
-                sigset_t newblocked;
-                /* A set of now blocked but previously unblocked signals. */
-                signandsets(&newblocked, newset, &current->blocked);
-                retarget_shared_pending(tsk, &newblocked);
-        }
-        tsk->blocked = *newset;
-        recalc_sigpending();
+        __set_task_blocked(tsk, newset);
         spin_unlock_irq(&tsk->sighand->siglock);
 }
 
@@ -2541,7 +2546,8 @@ int do_sigtimedwait(const sigset_t *which, siginfo_t *info,
                 /*
                  * None ready, temporarily unblock those we're interested
                  * while we are sleeping in so that we'll be awakened when
-                 * they arrive.
+                 * they arrive. Unblocking is always fine, we can avoid
+                 * set_current_blocked().
                  */
                 tsk->real_blocked = tsk->blocked;
                 sigandsets(&tsk->blocked, &tsk->blocked, &mask);
@@ -2551,10 +2557,9 @@ int do_sigtimedwait(const sigset_t *which, siginfo_t *info,
                 timeout = schedule_timeout_interruptible(timeout);
 
                 spin_lock_irq(&tsk->sighand->siglock);
-                sig = dequeue_signal(tsk, &mask, info);
-                tsk->blocked = tsk->real_blocked;
+                __set_task_blocked(tsk, &tsk->real_blocked);
                 siginitset(&tsk->real_blocked, 0);
-                recalc_sigpending();
+                sig = dequeue_signal(tsk, &mask, info);
         }
         spin_unlock_irq(&tsk->sighand->siglock);