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SubjectRe: [PATCH 1/2] block: generic request_queue reference counting
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On Sat, Oct 3, 2015 at 11:40 PM, Christoph Hellwig <hch@lst.de> wrote:
> On Tue, Sep 29, 2015 at 08:41:31PM -0400, Dan Williams wrote:
>> Allow pmem, and other synchronous/bio-based block drivers, to fallback
>> on a per-cpu reference count managed by the core for tracking queue
>> live/dead state.
>>
>> The existing per-cpu reference count for the blk_mq case is promoted to
>> be used in all block i/o scenarios. This involves initializing it by
>> default, waiting for it to drop to zero at exit, and holding a live
>> reference over the invocation of q->make_request_fn() in
>> generic_make_request(). The blk_mq code continues to take its own
>> reference per blk_mq request and retains the ability to freeze the
>> queue, but the check that the queue is frozen is moved to
>> generic_make_request().
>>
>> This fixes crash signatures like the following:
>>
>> BUG: unable to handle kernel paging request at ffff880140000000
>> [..]
>> Call Trace:
>> [<ffffffff8145e8bf>] ? copy_user_handle_tail+0x5f/0x70
>> [<ffffffffa004e1e0>] pmem_do_bvec.isra.11+0x70/0xf0 [nd_pmem]
>> [<ffffffffa004e331>] pmem_make_request+0xd1/0x200 [nd_pmem]
>> [<ffffffff811c3162>] ? mempool_alloc+0x72/0x1a0
>> [<ffffffff8141f8b6>] generic_make_request+0xd6/0x110
>> [<ffffffff8141f966>] submit_bio+0x76/0x170
>> [<ffffffff81286dff>] submit_bh_wbc+0x12f/0x160
>> [<ffffffff81286e62>] submit_bh+0x12/0x20
>> [<ffffffff813395bd>] jbd2_write_superblock+0x8d/0x170
>> [<ffffffff8133974d>] jbd2_mark_journal_empty+0x5d/0x90
>> [<ffffffff813399cb>] jbd2_journal_destroy+0x24b/0x270
>> [<ffffffff810bc4ca>] ? put_pwq_unlocked+0x2a/0x30
>> [<ffffffff810bc6f5>] ? destroy_workqueue+0x225/0x250
>> [<ffffffff81303494>] ext4_put_super+0x64/0x360
>> [<ffffffff8124ab1a>] generic_shutdown_super+0x6a/0xf0
>>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Keith Busch <keith.busch@intel.com>
>> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> Suggested-by: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>> block/blk-core.c | 71 +++++++++++++++++++++++++++++++++++++------
>> block/blk-mq-sysfs.c | 6 ----
>> block/blk-mq.c | 80 ++++++++++++++----------------------------------
>> block/blk-sysfs.c | 3 +-
>> block/blk.h | 14 ++++++++
>> include/linux/blk-mq.h | 1 -
>> include/linux/blkdev.h | 2 +
>> 7 files changed, 102 insertions(+), 75 deletions(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 2eb722d48773..6062550baaef 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -554,19 +554,17 @@ void blk_cleanup_queue(struct request_queue *q)
>> * Drain all requests queued before DYING marking. Set DEAD flag to
>> * prevent that q->request_fn() gets invoked after draining finished.
>> */
>> - if (q->mq_ops) {
>> - blk_mq_freeze_queue(q);
>> - spin_lock_irq(lock);
>> - } else {
>> - spin_lock_irq(lock);
>> + blk_freeze_queue(q);
>> + spin_lock_irq(lock);
>> + if (!q->mq_ops)
>> __blk_drain_queue(q, true);
>> - }
>
> __blk_drain_queue really ought to be moved into blk_freeze_queue so it
> has equivlaent functionality for mq vs !mq. But maybe that can be a
> separate patch.

It's not clear why the cleanup path is gating __blk_drain_queue on
->mq_ops? __blk_drain_queue already gets called from
blkcg_init_queue(). Also, it seems blk_get_flush_queue() is already
handling the mq_ops vs !mp_ops case, but I'd have to take a closer
look. Definitely seems like follow-on patch material...


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