Re: [PATCH] block: ensure the memory order between bi_private and bi_status

[Hou Tao <houtao1@huawei.com>]

ping ?

On 7/7/2021 2:29 PM, Hou Tao wrote:
> ping ?
>
> On 7/1/2021 7:35 PM, Hou Tao wrote:
>> When running stress test on null_blk under linux-4.19.y, the following
>> warning is reported:
>>
>>   percpu_ref_switch_to_atomic_rcu: percpu ref (css_release) <= 0 (-3) after switching to atomic
>>
>> The cause is that css_put() is invoked twice on the same bio as shown below:
>>
>> CPU 1:                         CPU 2:
>>
>> // IO completion kworker       // IO submit thread
>>                                __blkdev_direct_IO_simple
>>                                  submit_bio
>>
>> bio_endio
>>   bio_uninit(bio)
>>     css_put(bi_css)
>>     bi_css = NULL
>>                                set_current_state(TASK_UNINTERRUPTIBLE)
>>   bio->bi_end_io
>>     blkdev_bio_end_io_simple
>>       bio->bi_private = NULL
>>                                // bi_private is NULL
>>                                READ_ONCE(bio->bi_private)
>>         wake_up_process
>>           smp_mb__after_spinlock
>>
>>                                bio_unint(bio)
>>                                  // read bi_css as no-NULL
>>                                  // so call css_put() again
>>                                  css_put(bi_css)
>>
>> Because there is no memory barriers between the reading and the writing of
>> bi_private and bi_css, so reading bi_private as NULL can not guarantee
>> bi_css will also be NULL on weak-memory model host (e.g, ARM64).
>>
>> For the latest kernel source, css_put() has been removed from bio_unint(),
>> but the memory-order problem still exists, because the order between
>> bio->bi_private and {bi_status|bi_blkg} is also assumed in
>> __blkdev_direct_IO_simple(). It is reproducible that
>> __blkdev_direct_IO_simple() may read bi_status as 0 event if
>> bi_status is set as an errno in req_bio_endio().
>>
>> In __blkdev_direct_IO(), the memory order between dio->waiter and
>> dio->bio.bi_status is not guaranteed neither. Until now it is unable to
>> reproduce it, maybe because dio->waiter and dio->bio.bi_status are
>> in the same cache-line. But it is better to add guarantee for memory
>> order.
>>
>> Fixing it by using smp_load_acquire() & smp_store_release() to guarantee
>> the order between {bio->bi_private|dio->waiter} and {bi_status|bi_blkg}.
>>
>> Fixes: 189ce2b9dcc3 ("block: fast-path for small and simple direct I/O requests")
>> Signed-off-by: Hou Tao <houtao1@huawei.com>
>> ---
>>  fs/block_dev.c | 19 +++++++++++++++----
>>  1 file changed, 15 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index eb34f5c357cf..a602c6315b0b 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -224,7 +224,11 @@ static void blkdev_bio_end_io_simple(struct bio *bio)
>>  {
>>  	struct task_struct *waiter = bio->bi_private;
>>  
>> -	WRITE_ONCE(bio->bi_private, NULL);
>> +	/*
>> +	 * Paired with smp_load_acquire in __blkdev_direct_IO_simple()
>> +	 * to ensure the order between bi_private and bi_xxx
>> +	 */
>> +	smp_store_release(&bio->bi_private, NULL);
>>  	blk_wake_io_task(waiter);
>>  }
>>  
>> @@ -283,7 +287,8 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
>>  	qc = submit_bio(&bio);
>>  	for (;;) {
>>  		set_current_state(TASK_UNINTERRUPTIBLE);
>> -		if (!READ_ONCE(bio.bi_private))
>> +		/* Refer to comments in blkdev_bio_end_io_simple() */
>> +		if (!smp_load_acquire(&bio.bi_private))
>>  			break;
>>  		if (!(iocb->ki_flags & IOCB_HIPRI) ||
>>  		    !blk_poll(bdev_get_queue(bdev), qc, true))
>> @@ -353,7 +358,12 @@ static void blkdev_bio_end_io(struct bio *bio)
>>  		} else {
>>  			struct task_struct *waiter = dio->waiter;
>>  
>> -			WRITE_ONCE(dio->waiter, NULL);
>> +			/*
>> +			 * Paired with smp_load_acquire() in
>> +			 * __blkdev_direct_IO() to ensure the order between
>> +			 * dio->waiter and bio->bi_xxx
>> +			 */
>> +			smp_store_release(&dio->waiter, NULL);
>>  			blk_wake_io_task(waiter);
>>  		}
>>  	}
>> @@ -478,7 +488,8 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>>  
>>  	for (;;) {
>>  		set_current_state(TASK_UNINTERRUPTIBLE);
>> -		if (!READ_ONCE(dio->waiter))
>> +		/* Refer to comments in blkdev_bio_end_io */
>> +		if (!smp_load_acquire(&dio->waiter))
>>  			break;
>>  
>>  		if (!(iocb->ki_flags & IOCB_HIPRI) ||
> .