rcu: fix possible bug in include/linux/pagemap.h

Quoted 15 lines · Lai Jiangshan

I read the code and comments written by Nick Piggin(mainly from e286781d), page_cache_get_speculative() is protected by rcu_read_lock(),
it may be preempted when preemptible RCU, so we must use get_page_unless_zero() in this situation.

In the days of e286781d, we only have CLASSIC_RCU and (old)PREEMPT_RCU, so "defined(CONFIG_CLASSIC_RCU)" means non-preemptible RCU and the code is correct.

In the days of b560d8ad, we only have TREE_RCU and TREE_PREEMPT_RCU, so "defined(CONFIG_TREE_RCU)" means non-preemptible RCU and the code is correct.

But in nowadays, we have TREE_RCU, TREE_PREEMPT_RCU, TINY_RCU and TINY_PREEMPT_RCU, so the "defined(CONFIG_TREE_RCU)" for non-preemptible RCU is incorrect, and it may causes bugs. we should use "!defined(CONFIG_PREEMPT_RCU)" for non-preemptible RCU code block.

Quoted 4 lines

CC Nick Piggin <[email protected]>

Cc Paul E. McKenney <[email protected]>

Reported-by WANG Cong <[email protected]>

Signed-off-by Lai Jiangshan <[email protected]>

Good catch!!!

Acked-by Paul E. McKenney <[email protected]>

Quoted 21 lines · Paul E. McKenney

Quoted 19 lines · Lai Jiangshan

I read the code and comments written by Nick Piggin(mainly from e286781d), page_cache_get_speculative() is protected by rcu_read_lock(),
it may be preempted when preemptible RCU, so we must use get_page_unless_zero() in this situation.

In the days of e286781d, we only have CLASSIC_RCU and (old)PREEMPT_RCU, so "defined(CONFIG_CLASSIC_RCU)" means non-preemptible RCU and the code is correct.

In the days of b560d8ad, we only have TREE_RCU and TREE_PREEMPT_RCU, so "defined(CONFIG_TREE_RCU)" means non-preemptible RCU and the code is correct.

But in nowadays, we have TREE_RCU, TREE_PREEMPT_RCU, TINY_RCU and TINY_PREEMPT_RCU, so the "defined(CONFIG_TREE_RCU)" for non-preemptible RCU is incorrect, and it may causes bugs. we should use "!defined(CONFIG_PREEMPT_RCU)" for non-preemptible RCU code block.

CC Nick Piggin <[email protected]>

Cc Paul E. McKenney <[email protected]>

Reported-by WANG Cong <[email protected]>

Signed-off-by Lai Jiangshan <[email protected]>

Quoted 4 lines

Good catch!!!

Acked-by Paul E. McKenney <[email protected]>

Sorry, I forgot to CC it to LKML. Added it.