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. -- Jens Axboe
