On Sun, Sep 08, 2024 at 09:07:04AM +0900, Damien Le Moal wrote:
> Commit af2814149883 ("block: freeze the queue in queue_attr_store")
> changed queue_attr_store() to always freeze a sysfs attribute queue
> before calling the attribute store() method, to ensure that no IOs are
> in-flight when an attribute value is being updated.
>
> However, this change created a potential deadlock situation for the
> scheduler queue attribute as changing the queue elevator with
> elv_iosched_store() can result in a call to request_module() if the user
> requested module is not already registered. If the file of the requested
> module is stored on the block device of the frozen queue, a deadlock
> will happen as the read operations triggered by request_module() will
> wait for the queue freeze to end.
>
> Solve this issue by introducing the load_module method in struct
> queue_sysfs_entry, and to calling this method function in
> queue_attr_store() before freezing the attribute queue.
> The macro definition QUEUE_RW_LOAD_MODULE_ENTRY() is added to define a
> queue sysfs attribute that needs loading a module.
>
> The definition of the scheduler atrribute is changed to using
> QUEUE_RW_LOAD_MODULE_ENTRY(), with the function
> elv_iosched_load_module() defined as the load_module method.
> elv_iosched_store() can then be simplified to remove the call to
> request_module().
>
> Reported-by: Richard W.M. Jones <rjones@xxxxxxxxxx>
> Reported-by: Jiri Jaburek <jjaburek@xxxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219166
> Fixes: af2814149883 ("block: freeze the queue in queue_attr_store")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Damien Le Moal <dlemoal@xxxxxxxxxx>
I tested this (using my "unreliable" test!), and it passed 5,000
iterations, so:
Tested-by: Richard W.M. Jones <rjones@xxxxxxxxxx>
Thanks,
Rich.
> ---
> block/blk-sysfs.c | 22 +++++++++++++++++++++-
> block/elevator.c | 21 +++++++++++++++------
> block/elevator.h | 2 ++
> 3 files changed, 38 insertions(+), 7 deletions(-)
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 60116d13cb80..e85941bec857 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 (*load_module)(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_RW_LOAD_MODULE_ENTRY(_prefix, _name) \
> +static struct queue_sysfs_entry _prefix##_entry = { \
> + .attr = { .name = _name, .mode = 0644 }, \
> + .show = _prefix##_show, \
> + .load_module = _prefix##_load_module, \
> + .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_RW_LOAD_MODULE_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 +679,17 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
> if (!entry->store)
> return -EIO;
>
> + /*
> + * If the attribute needs to load a module, do it before freezing the
> + * queue to ensure that the module file can be read when the request
> + * queue is the one for the device storing the module file.
> + */
> + if (entry->load_module) {
> + res = entry->load_module(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..c355b55d0107 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_load_module(struct gendisk *disk, const char *buf,
> + size_t count)
> +{
> + char elevator_name[ELV_NAME_MAX];
> +
> + if (!elv_support_iosched(disk->queue))
> + return -EOPNOTSUPP;
> +
> + strscpy(elevator_name, buf, sizeof(elevator_name));
> +
> + return request_module("%s-iosched", strstrip(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..2a78544bf201 100644
> --- a/block/elevator.h
> +++ b/block/elevator.h
> @@ -148,6 +148,8 @@ extern void elv_unregister(struct elevator_type *);
> * io scheduler sysfs switching
> */
> ssize_t elv_iosched_show(struct gendisk *disk, char *page);
> +int elv_iosched_load_module(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 *);
> --
> 2.46.0
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit
