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 > 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); > ssize_t (*store)(struct gendisk *disk, const char *page, size_t count); > }; > > @@ -413,6 +414,14 @@ static struct queue_sysfs_entry _prefix##_entry = { \ > .store = _prefix##_store, \ > }; > > +#define QUEUE_RPW_ENTRY(_prefix, _name) \ > +static struct queue_sysfs_entry _prefix##_entry = { \ > + .attr = { .name = _name, .mode = 0644 }, \ > + .show = _prefix##_show, \ > + .pre_store = _prefix##_pre_store, \ > + .store = _prefix##_store, \ > +}; > + > QUEUE_RW_ENTRY(queue_requests, "nr_requests"); > QUEUE_RW_ENTRY(queue_ra, "read_ahead_kb"); > QUEUE_RW_ENTRY(queue_max_sectors, "max_sectors_kb"); > @@ -420,7 +429,7 @@ QUEUE_RO_ENTRY(queue_max_hw_sectors, "max_hw_sectors_kb"); > QUEUE_RO_ENTRY(queue_max_segments, "max_segments"); > QUEUE_RO_ENTRY(queue_max_integrity_segments, "max_integrity_segments"); > QUEUE_RO_ENTRY(queue_max_segment_size, "max_segment_size"); > -QUEUE_RW_ENTRY(elv_iosched, "scheduler"); > +QUEUE_RPW_ENTRY(elv_iosched, "scheduler"); > > QUEUE_RO_ENTRY(queue_logical_block_size, "logical_block_size"); > QUEUE_RO_ENTRY(queue_physical_block_size, "physical_block_size"); > @@ -670,6 +679,12 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr, > if (!entry->store) > return -EIO; > > + if (entry->pre_store) { > + res = entry->pre_store(disk, page, length); > + if (res) > + return res; > + } > + > blk_mq_freeze_queue(q); > mutex_lock(&q->sysfs_lock); > res = entry->store(disk, page, length); > diff --git a/block/elevator.c b/block/elevator.c > index f13d552a32c8..c338282d5148 100644 > --- a/block/elevator.c > +++ b/block/elevator.c > @@ -698,17 +698,26 @@ static int elevator_change(struct request_queue *q, const > char *elevator_name) > return 0; > > e = elevator_find_get(q, elevator_name); > - if (!e) { > - request_module("%s-iosched", elevator_name); > - e = elevator_find_get(q, elevator_name); > - if (!e) > - return -EINVAL; > - } > + if (!e) > + return -EINVAL; > ret = elevator_switch(q, e); > elevator_put(e); > return ret; > } > > +int elv_iosched_pre_store(struct gendisk *disk, const char *buf, > + size_t count) > +{ > + char elevator_name[ELV_NAME_MAX]; > + > + if (!elv_support_iosched(disk->queue)) > + return -ENOTSUPP; > + > + strscpy(elevator_name, buf, sizeof(elevator_name)); > + > + return request_module("%s-iosched", elevator_name); > +} > + > ssize_t elv_iosched_store(struct gendisk *disk, const char *buf, > size_t count) > { > diff --git a/block/elevator.h b/block/elevator.h > index 3fe18e1a8692..059172c0f93c 100644 > --- a/block/elevator.h > +++ b/block/elevator.h > @@ -148,6 +148,7 @@ extern void elv_unregister(struct elevator_type *); > * io scheduler sysfs switching > */ > ssize_t elv_iosched_show(struct gendisk *disk, char *page); > +int elv_iosched_pre_store(struct gendisk *disk, const char *page, size_t count); > ssize_t elv_iosched_store(struct gendisk *disk, const char *page, size_t count); > > extern bool elv_bio_merge_ok(struct request *, struct bio *); I tested this on top of current git head and it fixes the problem for me. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
