Subject: rcu: rcutiny: Prevent RCU stall From: Thomas Gleixner Date: Tue, 16 Oct 2012 18:36:51 +0200 rcu_read_unlock_special() checks in_serving_softirq() and leaves early when true. On RT this is obviously wrong as softirq processing context can be preempted and therefor such a task can be on the gp_tasks list. Leaving early here will leave the task on the list and therefor block RCU processing forever. This cannot happen on mainline because softirq processing context cannot be preempted and therefor this can never happen at all. In fact this check looks quite questionable in general. Neither irq context nor softirq processing context in mainline can ever be preempted in mainline so the special unlock case should not ever be invoked in such context. Now the only explanation might be a rcu_read_unlock() being interrupted and therefor leave the rcu nest count at 0 before the special unlock bit has been cleared. That looks fragile. At least it's missing a big fat comment. Paul ???? See mainline commits: ec433f0c5 and 8762705a for further enlightment. Reported-by: Kristian Lehmann Signed-off-by: Thomas Gleixner Cc: stable-rt@vger.kernel.org --- kernel/rcutiny_plugin.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) Index: linux-stable/kernel/rcutiny_plugin.h =================================================================== --- linux-stable.orig/kernel/rcutiny_plugin.h +++ linux-stable/kernel/rcutiny_plugin.h @@ -560,7 +560,7 @@ void rcu_read_unlock_special(struct task rcu_preempt_cpu_qs(); /* Hardware IRQ handlers cannot block. */ - if (in_irq() || in_serving_softirq()) { + if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) { local_irq_restore(flags); return; }