Re: [PATCH] block: Prevent deadlocks when switching elevators

["Richard W.M. Jones" <rjones@redhat.com>]

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@redhat.com>
> Reported-by: Jiri Jaburek <jjaburek@redhat.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219166
> Fixes: af2814149883 ("block: freeze the queue in queue_attr_store")
> Cc: stable@vger.kernel.org
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

I tested this (using my "unreliable" test!), and it passed 5,000
iterations, so:

Tested-by: Richard W.M. Jones <rjones@redhat.com>

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
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