On Sat, Sep 07, 2024 at 05:48:44PM +0800, Ming Lei wrote: > On Sat, Sep 07, 2024 at 06:04:59PM +0900, 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 > > Right, I remember that the freezing starts to cover update of > max_sectors_kb. > > > be to do the request_module() before freezing the queue. > > Something like below should work (totally untested and that may be overkill). > > > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c > > index 60116d13cb80..aef87f6b4a8a 100644 > > --- a/block/blk-sysfs.c > > +++ b/block/blk-sysfs.c > > @@ -23,6 +23,7 @@ > > struct queue_sysfs_entry { > > struct attribute attr; > > ssize_t (*show)(struct gendisk *disk, char *page); > > + int (*pre_store)(struct gendisk *disk, const char *page, size_t count); > > It seems over-kill to add one new callback, and another way is just to > not freeze queue for storing elevator. > > But if other attribute update needs to not freeze queue, 'pre_store' > looks one reasonable solution. > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c > index 60116d13cb80..c418edf66f0c 100644 > --- a/block/blk-sysfs.c > +++ b/block/blk-sysfs.c > @@ -666,15 +666,24 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr, > struct gendisk *disk = container_of(kobj, struct gendisk, queue_kobj); > struct request_queue *q = disk->queue; > ssize_t res; > + bool need_freeze; > > if (!entry->store) > return -EIO; > > - blk_mq_freeze_queue(q); > + /* > + * storing scheduler freezes queue in its way, especially > + * loading scheduler module can't be done when queue is frozen > + */ > + need_freeze = (entry->store == elv_iosched_store); > + > + if (need_freeze) > + blk_mq_freeze_queue(q); > mutex_lock(&q->sysfs_lock); > res = entry->store(disk, page, length); > mutex_unlock(&q->sysfs_lock); > - blk_mq_unfreeze_queue(q); > + if (need_freeze) > + blk_mq_unfreeze_queue(q); > return res; > } > Unfortunately this doesn't fix the problem for me. The test still hangs occasionally in the same way as before. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html
