On 02/27/2017 09:25 AM, Omar Sandoval wrote: > On Mon, Feb 27, 2017 at 09:15:27AM -0700, Jens Axboe wrote: >> On 02/27/2017 09:10 AM, Sagi Grimberg wrote: >>> >>>>> Hm, this may fix the crash, but I'm not sure it'll work as intended. >>>>> When we allocate the request, we'll get a reserved scheduler tag, but >>>>> then when we go to dispatch the request and call >>>>> blk_mq_get_driver_tag(), we'll be competing with all of the normal >>>>> requests for a regular driver tag. So maybe on top of this we should add >>>>> the BLK_MQ_REQ_RESERVED flag to the allocation attempt in >>>>> blk_mq_get_driver_tag() if the scheduler tag is reserved? I'm hazy on >>>>> what we expect from reserved tags, so feel free to call me crazy. >>>> >>>> Yeah good point, we need to carry it through. Reserved tags exist >>>> because drivers often need a request/tag for error handling. If all >>>> tags currently are used up for regular IO that is stuck, you need >>>> a reserved tag for error handling to guarantee progress. >>>> >>>> So Sagi's patch does take it half the way there, but get_driver_tag >>>> really needs to know about this as well, or we will just get stuck >>>> there as well. Two solutions, I can think of: >>>> >>>> 1) Check the tag value in get_driver_tag, add BLK_MQ_REQ_RESERVED >>>> when allocating a driver tag if above X. >>>> 2) Add an RQF_SOMETHING_RESERVED. Add BLK_MQ_REQ_RESERVED in >>>> get_driver_tag if that is set. >>>> >>>> Comments? >>> >>> Can't we just not go through the scheduler for reserved tags? Obviously >>> there is no point in scheduling them... >> >> Right, that would be possible. But I'd rather not treat any requests >> differently, it's a huge pain in the ass that flush request currently >> insert with a driver tag already allocated. So it's not because >> scheduling will add anything at all, it's more that I'd like to move >> flush requests to use regular inserts as well and not deal with some >> request being "special" in any way. >> >> The below should hopefully work. Totally untested... > > I like your variant if it works for Sagi. My only complaint (which was > already there) is that the BUG_ON(tag >= tags->nr_reserved_tags) in > blk_mq_put_tag() looks kind of silly since we just checked that exact > same condition. Yeah, that check is nonsensical. Let's kill it. -- Jens Axboe
