Re: [PATCH 1/2] Avoid bio_endio recursion

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 25 Jun 2008, Jens Axboe wrote:

> On Tue, Jun 24 2008, Mikulas Patocka wrote:
>>
>>
>> On Tue, 24 Jun 2008, Jens Axboe wrote:
>>
>>> On Tue, Jun 24 2008, Mikulas Patocka wrote:
>>>> Hi
>>>>
>>>> bio_endio calls bi_end_io callback. In case of stacked devices (raid, dm),
>>>> bio_end_io may call bio_endio again, up to an unspecified length.
>>>>
>>>> The crash because of stack overflow was really observed on sparc64. And
>>>> this recursion was one of the contributing factors (using 9 stack frames
>>>> --- that is 1728 bytes).
>>>
>>> Looks good, I like the concept. Can you please make it a little less
>>> goto driven, though? The next_bio and goto next_bio could just be a
>>> while().
>>>
>>> --
>>> Jens Axboe
>>>
>>
>> Hi.
>>
>> This is the patch, slightly de-goto-ized. (it still contains one, I think
>> that while (1) { ... break ... } is no better readable than goto).
>
> Sure, that looks better.
>
>> I found another problem in my previous patch, I forgot about the "error"
>> variable (it would cause misbehavior for example if disk fails, submits an
>> error and raid driver turns this failure into success). We need to save
>> the error variable somewhere in the bio, there is no other place where it
>> could be placed. I temporarily saved it to bi_idx, because it's unused at
>> this place.
>
> I don't think bi_idx is a fantastic idea, I could easily imagine the
> bi_end_io function wanting to do a segment loop on the bio. Use
> bi_phys_segments instead (or bi_hw_segemnts, no difference), they should
> only be used when queuing and building IO, not for completion purposes.
> And put a big fat comment there explaining the overload. Plus they are
> just a cache, so if you use either of those and at the same time clear
> BIO_SEG_VALID in bi_flags, then it's guarenteed to be safe.

Here is new patch that uses bi_phys_segments.

> Also please put the per-cpu definition outside of bio_endio(). And I
> don't think you need to disable interrupts, a plain preempt_disable() /
> preempt_enable() should be enough.

Disabling interrupts is needed. If you disable only preempt, interrupt can 
come at any point and add anything to the queue. You'd have to use local_t 
and local_cmpxchg tricks then and the code would be much more ugly. I 
found that the code is called with interrupts disabled most time, so it 
doesn't matter if I disable them again.

Mikulas

> -- 
> Jens Axboe

Avoid recursion on bio_endio. bio_endio calls bio->bi_end_io which may in turn
call bio_endio again. When this recursion happens, put the new bio to the queue
and process it later, from the top-level bio_endio.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
  fs/bio.c            |   38 +++++++++++++++++++++++++++++++++++---
  include/linux/bio.h |    3 +++
  2 files changed, 38 insertions(+), 3 deletions(-)

Index: linux-2.6.26-rc7-devel/fs/bio.c
===================================================================
--- linux-2.6.26-rc7-devel.orig/fs/bio.c	2008-06-23 13:49:45.000000000 +0200
+++ linux-2.6.26-rc7-devel/fs/bio.c	2008-06-25 18:09:48.000000000 +0200
@@ -1166,15 +1166,47 @@ void bio_check_pages_dirty(struct bio *b
   *   bio unless they own it and thus know that it has an end_io
   *   function.
   **/
+static DEFINE_PER_CPU(struct bio **, bio_end_queue) = { NULL };
+
  void bio_endio(struct bio *bio, int error)
  {
+	struct bio ***bio_end_queue_ptr;
+	struct bio *bio_queue;
+
+	unsigned long flags;
+
+	bio->bi_phys_segments = error;
  	if (error)
  		clear_bit(BIO_UPTODATE, &bio->bi_flags);
  	else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
-		error = -EIO;
+		bio->bi_phys_segments = -EIO;
+
+	local_irq_save(flags);
+	bio_end_queue_ptr = &__get_cpu_var(bio_end_queue);
+
+	if (*bio_end_queue_ptr) {
+		**bio_end_queue_ptr = bio;
+		*bio_end_queue_ptr = &bio->bi_next;
+		bio->bi_next = NULL;
+	} else {
+		bio_queue = NULL;
+		*bio_end_queue_ptr = &bio_queue;
+
+next_bio:
+		if (bio->bi_end_io)
+			bio->bi_end_io(bio, (short)bio->bi_phys_segments);
+
+		if (bio_queue) {
+			bio = bio_queue;
+			bio_queue = bio->bi_next;
+			if (!bio_queue)
+				*bio_end_queue_ptr = &bio_queue;
+			goto next_bio;
+		}
+		*bio_end_queue_ptr = NULL;
+	}

-	if (bio->bi_end_io)
-		bio->bi_end_io(bio, error);
+	local_irq_restore(flags);
  }

  void bio_pair_release(struct bio_pair *bp)
Index: linux-2.6.26-rc7-devel/include/linux/bio.h
===================================================================
--- linux-2.6.26-rc7-devel.orig/include/linux/bio.h	2008-06-25 18:06:59.000000000 +0200
+++ linux-2.6.26-rc7-devel/include/linux/bio.h	2008-06-25 18:08:08.000000000 +0200
@@ -86,6 +86,9 @@ struct bio {

  	/* Number of segments in this BIO after
  	 * physical address coalescing is performed.
+	 *
+	 * When ending a bio request in bio_endio, this field is temporarily
+	 * (ab)used to keep the error code.
  	 */
  	unsigned short		bi_phys_segments;