rcu: Eliminate signed overflow in synchronize_rcu_expedited() (bcfa57ce) · Commits · 戴 / test

In the C language, signed overflow is undefined. It is true that twos-complement arithmetic normally comes to the rescue, but if the compiler can subvert this any time it has any information about the values being compared. For example, given "if (a - b > 0)", if the compiler has enough information to realize that (for example) the value of "a" is positive and that of "b" is negative, the compiler is within its rights to optimize to a simple "if (1)", which might not be what you want. This commit therefore converts synchronize_rcu_expedited()'s work-done detection counter from signed to unsigned. Signed-off-by:

Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by:

Josh Triplett <josh@joshtriplett.org>

kernel/rcutree_plugin.h

+4 −4

Original line number	Diff line number	Diff line
		@@ -677,7 +677,7 @@ void synchronize_rcu(void)
		EXPORT_SYMBOL_GPL(synchronize_rcu);

		static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
		static long sync_rcu_preempt_exp_count;
		static unsigned long sync_rcu_preempt_exp_count;
		static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);

		/*
		@@ -792,7 +792,7 @@ void synchronize_rcu_expedited(void)
		unsigned long flags;
		struct rcu_node *rnp;
		struct rcu_state *rsp = &rcu_preempt_state;
		long snap;
		unsigned long snap;
		int trycount = 0;

		smp_mb(); /* Caller's modifications seen first by other CPUs. */
		@@ -811,10 +811,10 @@ void synchronize_rcu_expedited(void)
		synchronize_rcu();
		return;
		}
		if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
		if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
		goto mb_ret; /* Others did our work for us. */
		}
		if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
		if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
		goto unlock_mb_ret; /* Others did our work for us. */

		/* force all RCU readers onto ->blkd_tasks lists. */

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