On Mon, Sep 09, 2024 at 10:59:00AM +0900, Damien Le Moal wrote: > On 9/9/24 10:24, Ming Lei wrote: > > On Sat, Sep 07, 2024 at 07:50:32AM -0600, Jens Axboe wrote: > >> On 9/7/24 3:04 AM, Damien Le Moal wrote: > >>> On 9/7/24 16:58, Ming Lei wrote: > >>>> On Sat, Sep 07, 2024 at 08:35:22AM +0100, Richard W.M. Jones wrote: > >>>>> On Sat, Sep 07, 2024 at 09:43:31AM +0800, Ming Lei wrote: > >>>>>> When switching io scheduler via sysfs, 'request_module' may be called > >>>>>> if the specified scheduler doesn't exist. > >>>>>> > >>>>>> This was has deadlock risk because the module may be stored on FS behind > >>>>>> our disk since request queue is frozen before switching its elevator. > >>>>>> > >>>>>> Fix it by returning -EDEADLK in case that the disk is claimed, which > >>>>>> can be thought as one signal that the disk is mounted. > >>>>>> > >>>>>> Some distributions(Fedora) simulates the original kernel command line of > >>>>>> 'elevator=foo' via 'echo foo > /sys/block/$DISK/queue/scheduler', and boot > >>>>>> hang is triggered. > >>>>>> > >>>>>> Cc: Richard Jones <rjones@xxxxxxxxxx> > >>>>>> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx> > >>>>>> Cc: Jiri Jaburek <jjaburek@xxxxxxxxxx> > >>>>>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > >>>>> > >>>>> I'd suggest also: > >>>>> > >>>>> Bug: https://bugzilla.kernel.org/show_bug.cgi?id=219166 > >>>>> Reported-by: Richard W.M. Jones <rjones@xxxxxxxxxx> > >>>>> Reported-by: Jiri Jaburek <jjaburek@xxxxxxxxxx> > >>>>> Tested-by: Richard W.M. Jones <rjones@xxxxxxxxxx> > >>>>> > >>>>> So I have tested this patch and it does fix the issue, at the possible > >>>>> cost that now setting the scheduler can fail: > >>>>> > >>>>> + for f in /sys/block/{h,s,ub,v}d*/queue/scheduler > >>>>> + echo noop > >>>>> /init: line 109: echo: write error: Resource deadlock avoided > >>>>> > >>>>> (I know I'm setting it to an impossible value here, but this could > >>>>> also happen when setting it to a valid one.) > >>>> > >>>> Actually in most of dist, io-schedulers are built-in, so request_module > >>>> is just a nop, but meta IO must be started. > >>>> > >>>>> > >>>>> Since almost no one checks the result of 'echo foo > /sys/...' that > >>>>> would probably mean that sometimes a desired setting is silently not > >>>>> set. > >>>> > >>>> As I mentioned, io-schedulers are built-in for most of dist, so > >>>> request_module isn't called in case of one valid io-sched. > >>>> > >>>>> > >>>>> Also I bisected this bug yesterday and found it was caused by (or, > >>>>> more likely, exposed by): > >>>>> > >>>>> commit af2814149883e2c1851866ea2afcd8eadc040f79 > >>>>> Author: Christoph Hellwig <hch@xxxxxx> > >>>>> Date: Mon Jun 17 08:04:38 2024 +0200 > >>>>> > >>>>> block: freeze the queue in queue_attr_store > >>>>> > >>>>> queue_attr_store updates attributes used to control generating I/O, and > >>>>> can cause malformed bios if changed with I/O in flight. Freeze the queue > >>>>> in common code instead of adding it to almost every attribute. > >>>>> > >>>>> Reverting this commit on top of git head also fixes the problem. > >>>>> > >>>>> Why did this commit expose the problem? > >>>> > >>>> That is really the 1st bad commit which moves queue freezing before > >>>> calling request_module(), originally we won't freeze queue until > >>>> we have to do it. > >>>> > >>>> Another candidate fix is to revert it, or at least not do it > >>>> for storing elevator attribute. > >>> > >>> I do not think that reverting is acceptable. Rather, a proper fix would simply > >>> be to do the request_module() before freezing the queue. > >>> Something like below should work (totally untested and that may be overkill). > >> > >> I like this approach, but let's please call it something descriptive > >> like "load_module" or something like that. > > > > But 'load_module' is too specific as interface, and we just only have > > one case which need to load module exactly. > > If another attr needs to do some prep work before freezing the queue and calling > attr->store(), we can rename the load_module attribute method to something like > "prepare_store" to be more generic. 'interface' is supposed to be generic from beginning, and I don't think we will have another 'load_module' case here. Thanks, Ming
