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. Thanks, Ming
