signal: sigprocmask: narrow the scope of ->siglock

No functional changes, preparation to simplify the review of the next change.

1. We can read current->block lockless, nobody else can ever change this mask.

2. Calculate the resulting sigset_t outside of ->siglock into the temporary
   variable, then take ->siglock and change ->blocked.

Also, kill the stale comment about BKL.

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

1 files changed, 13 insertions, 16 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 707736e64e0e..e8308e3238c1 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2300,12 +2300,6 @@ long do_no_restart_syscall(struct restart_block *param)
 }
 
 /*
- * We don't need to get the kernel lock - this is all local to this
- * particular thread.. (and that's good, because this is _heavily_
- * used by various programs)
- */
-
-/*
  * This is also useful for kernel threads that want to temporarily
  * (or permanently) block certain signals.
  *
@@ -2315,30 +2309,33 @@ long do_no_restart_syscall(struct restart_block *param)
  */
 int sigprocmask(int how, sigset_t *set, sigset_t *oldset)
 {
-        int error;
+        struct task_struct *tsk = current;
+        sigset_t newset;
 
-        spin_lock_irq(&current->sighand->siglock);
+        /* Lockless, only current can change ->blocked, never from irq */
         if (oldset)
-                *oldset = current->blocked;
+                *oldset = tsk->blocked;
 
-        error = 0;
         switch (how) {
         case SIG_BLOCK:
-                sigorsets(&current->blocked, &current->blocked, set);
+                sigorsets(&newset, &tsk->blocked, set);
                 break;
         case SIG_UNBLOCK:
-                signandsets(&current->blocked, &current->blocked, set);
+                signandsets(&newset, &tsk->blocked, set);
                 break;
         case SIG_SETMASK:
-                current->blocked = *set;
+                newset = *set;
                 break;
         default:
-                error = -EINVAL;
+                return -EINVAL;
         }
+
+        spin_lock_irq(&tsk->sighand->siglock);
+        tsk->blocked = newset;
         recalc_sigpending();
-        spin_unlock_irq(&current->sighand->siglock);
+        spin_unlock_irq(&tsk->sighand->siglock);
 
-        return error;
+        return 0;
 }
 
 /**