Re: [PATCH] signals: Avoid unnecessary taking of sighand->siglock

From: Waiman Long
Date: Mon Sep 26 2016 - 17:55:31 EST

On 09/23/2016 05:32 AM, Oleg Nesterov wrote:
On 09/22, Waiman Long wrote:
This patch is currently only active for 64-bit architectures.

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2485,6 +2485,16 @@ void __set_current_blocked(const sigset_t *newset)
struct task_struct *tsk = current;

+ /*
+ * In case the signal mask hasn't changed, we won't need to take
+ * the lock. As the current blocked mask can be modified by other
+ * CPUs,
No, nobody else should modify current->blocked.

Thanks for the clarification. I am just not 100% sure about that. I will modify the comment.

Yes, we need to cleanup the usage of force_sig_info(), and probably remove
the "struct task_struct *t" argument.

we need to do an atomic read without lock. In other words,
+ * this check will only be done on 64-bit systems.
+ */
+#if _NSIG_WORDS == 1
+ if (READ_ONCE(tsk->blocked.sig[0]) == newset->sig[0])
+ return;
OK, agreed, but this should not depend on _NSIG_WORDS == 1 and
READ_ONCE() looks confusing. It seems you need to add the new helper
into include/linux/signal.h.

Yes, I will add a helper into signal.h and call it instead.