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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-01-24 08:51:34 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-01-25 12:54:22 -0800
commit7f554a3d05bea9f6b7bf8e0b041d09447f82d74a (patch)
treed679d50032981ee84479dfaea34716ab1ac57d6d /kernel/rcu
parent418b2977b34378f67c46930c72a776f94e7bf903 (diff)
downloadlinux-7f554a3d05bea9f6b7bf8e0b041d09447f82d74a.tar.bz2
srcu: Reduce probability of SRCU ->unlock_count[] counter overflow
Because there are no memory barriers between the srcu_flip() ->completed increment and the summation of the read-side ->unlock_count[] counters, both the compiler and the CPU can reorder the summation with the ->completed increment. If the updater is preempted long enough during this process, the read-side counters could overflow, resulting in a too-short grace period. This commit therefore adds a memory barrier just after the ->completed increment, ensuring that if the summation misses an increment of ->unlock_count[] from __srcu_read_unlock(), the next __srcu_read_lock() will see the new value of ->completed, thus bounding the number of ->unlock_count[] increments that can be missed to NR_CPUS. The actual overflow computation is more complex due to the possibility of nesting of __srcu_read_lock(). Reported-by: Lance Roy <ldr709@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/srcu.c11
1 files changed, 10 insertions, 1 deletions
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 665bc9951523..e773129c8b08 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -320,7 +320,16 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
*/
static void srcu_flip(struct srcu_struct *sp)
{
- sp->completed++;
+ WRITE_ONCE(sp->completed, sp->completed + 1);
+
+ /*
+ * Ensure that if the updater misses an __srcu_read_unlock()
+ * increment, that task's next __srcu_read_lock() will see the
+ * above counter update. Note that both this memory barrier
+ * and the one in srcu_readers_active_idx_check() provide the
+ * guarantee for __srcu_read_lock().
+ */
+ smp_mb(); /* D */ /* Pairs with C. */
}
/*