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Date: Mon, 7 Jul 2014 15:38:07 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
oleg@...hat.com, sbw@....edu,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 03/17] signal: Explain local_irq_save() call
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
The explicit local_irq_save() in __lock_task_sighand() is needed to avoid
a potential deadlock condition, as noted in a841796f11c90d53 (signal:
align __lock_task_sighand() irq disabling and RCU). However, someone
reading the code might be forgiven for concluding that this separate
local_irq_save() was completely unnecessary. This commit therefore adds
a comment referencing the shiny new block comment on rcu_read_unlock().
Reported-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Oleg Nesterov <oleg@...hat.com>
---
kernel/signal.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/kernel/signal.c b/kernel/signal.c
index a4077e90f19f..46161e744760 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1263,6 +1263,10 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
struct sighand_struct *sighand;
for (;;) {
+ /*
+ * Disable interrupts early to avoid deadlocks.
+ * See rcu_read_unlock comment header for details.
+ */
local_irq_save(*flags);
rcu_read_lock();
sighand = rcu_dereference(tsk->sighand);
--
1.8.1.5
--
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