Re: [PATCH] block: fix mis-synchronisation in blkdev_issue_zeroout()

[Jeff Moyer]

Lukas Czerner <lczerner@xxxxxxxxxx> writes:

> On Fri, 4 Mar 2011, Jeff Moyer wrote:
>
>> Lukas Czerner <lczerner@xxxxxxxxxx> writes:
>> 
>> > On Fri, 4 Mar 2011, Jeff Moyer wrote:
>> >> It seems to me like it might be better to just not complete anything
>> >> until the count is zero.  Why issue a wakeup for every bio?
>> >> fs/direct-io does something similar, maybe take a look at the
>> >> dio_bio_end* routines and see if that would fit well here.  With your
>> >> scheme, I worry about missing a completion, maybe because the first bio
>> >> completes before you are done submitting bios.  Is that possible?
>> >
>> > I do not think it is possible. For every bio submitted there is
>> > wait_for_completion called. When bio complete()s completion->done is
>> > incremented (under the wait->lock). In wait_for_completion() we are
>> > waiting for single submitted bio to complete (completion->done > 0),
>> > then completion->done is decremented. It seems like simple
>> > synchronization.
>> >
>> > I am not sure what wakeup you have in mind, but thanks for the tip I'll
>> > look in fs/direct-io.
>> 
>> Let's say you have several bios to submit, and the first bio is errored
>> immediately in submit_bio.  Since you didn't add yourself to the
>> waitqueue yet, you might miss the wakeup and sleep forever.
>> 
>> Cheers,
>> Jeff
>> 
>
> (Adding Dimitry and Jens into CC)
>
> This can not happen. submit_bio does not return any value. The way how
> it does notify caller about its status is via ->bio_end_io (see comment
> for __generic_make_request()). Now the ->bio_end_io is in this case
> always set because it is the first thing we are doing, so for every bio
> submitted there will be appropriate complete() and wait_for_completition()
> call.
>
> The one thing can fail though, and it is bio_alloc() however when this
> fails we are jumping out of the loop immediately without touching
> "issued" at all, so if it is a first bio issued = 0, hence there is
> nothing to wait for.
>
> I do not see any problems here.

Yeah, somehow I actually missed the whole point of completions (the done
variable).  Anyway, I agree with you.

> But, now I can see you point about calling wakeup() for every completed
> bio, which is not strictly needed and we should call complete() only
> once when the last bio is completed. So how about this ?
>
>
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 1a320d2..ccf5a40 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -109,7 +109,6 @@ struct bio_batch
>  	atomic_t 		done;
>  	unsigned long 		flags;
>  	struct completion 	*wait;
> -	bio_end_io_t		*end_io;
>  };
>  
>  static void bio_batch_end_io(struct bio *bio, int err)
> @@ -122,12 +121,9 @@ static void bio_batch_end_io(struct bio *bio, int err)
>  		else
>  			clear_bit(BIO_UPTODATE, &bb->flags);
>  	}
> -	if (bb) {
> -		if (bb->end_io)
> -			bb->end_io(bio, err);
> -		atomic_inc(&bb->done);
> -		complete(bb->wait);
> -	}
> +	if (bb)
> +		if (atomic_dec_and_test(&bb->done))
> +			complete(bb->wait);

I don't think we actually have to check for bb != NULL, do you?

>  	bio_put(bio);
>  }
>  
> @@ -150,13 +146,12 @@ int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>  	int ret;
>  	struct bio *bio;
>  	struct bio_batch bb;
> -	unsigned int sz, issued = 0;
> +	unsigned int sz;
>  	DECLARE_COMPLETION_ONSTACK(wait);
>  
> -	atomic_set(&bb.done, 0);
> +	atomic_set(&bb.done, 1);
>  	bb.flags = 1 << BIO_UPTODATE;
>  	bb.wait = &wait;
> -	bb.end_io = NULL;
>  
>  submit:
>  	ret = 0;
> @@ -185,12 +180,12 @@ submit:
>  				break;
>  		}
>  		ret = 0;
> -		issued++;
> +		atomic_inc(&bb.done);
>  		submit_bio(WRITE, bio);
>  	}
>  
>  	/* Wait for bios in-flight */
> -	while (issued != atomic_read(&bb.done))
> +	if (!atomic_dec_and_test(&bb.done))
>  		wait_for_completion(&wait);
>  
>  	if (!test_bit(BIO_UPTODATE, &bb.flags))

Yep, this looks good to me.  Thanks for fixing this up, Lukas!

Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
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