Re: [PATCH] loop: defer the queue limits clear to a workqueue

[Tao Cui <cui.tao@linux.dev>]

在 2026/9/24 18:20, Tao Cui 写道:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> loop_clear_limits() calls queue_limits_commit_update() directly from
> the loop workqueue that processes the request.  That does a
> non-atomic struct assignment to q->limits without freezing the queue,
> which races with lockless readers of q->limits on other CPUs - bio
> splitting reads max_hw_sectors, the discard path reads
> max_hw_discard_sectors - and can let them observe torn values.  The
> trigger is a discard or write-zeroes request on a loop device whose
> backing file does not support the corresponding fallocate operation.
> 

Oops, I messed up the subject, that was supposed to be [PATCH v5].
The patch in it is the final version: rebased as a single commit,
plus one fix over v4, clear_limits_lock now covers the limits
commit too, since the queue freeze is counted and doesn't exclude
a concurrent loop_change_fd(). Please ignore the subject and treat
it as v5.

Sorry for the noise,
Tao.

> The code already has an XXX comment saying this should move to a
> workqueue.  Do that: schedule a work item on the system workqueue, where
> it is safe to freeze the queue around the limits update.  The pending
> modes and the rebind generation live under a new mutex,
> lo->clear_limits_lock.  The work item takes the mutex with the queue
> frozen and holds it over the limits commit, so a rebind cannot slip
> in between the generation check and the commit.  Rebinding the
> device drops the accumulated modes and invalidates an already
> scheduled work item, so a stale clear cannot hit the new backing
> file.  The work item is cancelled before the device is freed.
> 
> Suggested-by: Bart Van Assche <bvanassche@acm.org>
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> 
> ---
> Changes since v4:
> 
> - Rebase onto the current block tree as a single commit.
> 
> - Hold clear_limits_lock over the limits commit in the work item.
>   The queue freeze is counted, not mutually exclusive, so
>   loop_change_fd() could rebind between the generation check and
>   the commit, and a stale clear could disable discard and write
>   zeroes on the new backing file.
> 
> Changes since v3:
> 
> - Replace the three atomic variables (clear_limits_mode, rebind_gen,
>   clear_limits_gen) with plain variables protected by a new
>   clear_limits_lock mutex, as suggested by Bart.  The mutex is taken
>   after the queue freeze in the work item, which keeps the same
>   freeze -> mutex ordering as loop_change_fd(), the only rebinding
>   path that freezes the queue.
> 
> Changes since v2:
> 
> - Skip the clear when the device was rebound since the work item
>   was scheduled: the modes used to be captured before the freeze,
>   so a LOOP_CHANGE_FD completing in between could apply the old
>   modes to the new backing file.  The rebind generation is now
>   checked with the queue frozen, which excludes loop_change_fd()
>   because it assigns the new backing file under the same freeze,
>   so a rebind cannot slip in between the check and the commit.
>   Cancelling from loop_assign_backing_file() would instead
>   deadlock on the freeze held by loop_change_fd().
> 
> - Also bump the rebind generation from __loop_clr_fd(): unbinding
>   does not go through loop_assign_backing_file(), so a work item
>   scheduled before the last close could otherwise commit a stale
>   clear to the queue limits of the unbound device.
> 
> Changes since v1:
> 
> - Reset clear_limits_mode when assigning a new backing file, so
>   stale modes do not clear limits of the new file.
> 
> - Cancel the work item from loop_remove() before del_gendisk():
>   the queue can already be in RCU-delayed freeing when
>   lo_free_disk() cancels it.
> 
> Tested on x86-64 (qemu, vfat-backed loop device): a 30s discard and
> reconfigure loop exercises the clear path 288 times, no torn sysfs
> reads, no difference against the unpatched kernel.
> 
> Link: https://lore.kernel.org/r/20260828072004.273519-1-cui.tao@linux.dev/
> ---
>  drivers/block/loop.c | 83 ++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 69 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 758c20678bf6c..0b89036c982ad 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -67,6 +67,11 @@ struct loop_device {
>  	struct list_head        rootcg_cmd_list;
>  	struct list_head        idle_worker_list;
>  	struct rb_root          worker_tree;
> +	struct work_struct      clear_limits_work;
> +	struct mutex		clear_limits_lock;
> +	unsigned int		clear_limits_mode;
> +	unsigned int		rebind_gen;
> +	unsigned int		clear_limits_gen;
>  	struct timer_list       timer;
>  	bool			sysfs_inited;
>  
> @@ -222,26 +227,53 @@ static void loop_set_size(struct loop_device *lo, loff_t size)
>  		kobject_uevent(&disk_to_dev(lo->lo_disk)->kobj, KOBJ_CHANGE);
>  }
>  
> -static void loop_clear_limits(struct loop_device *lo, int mode)
> +static void loop_clear_limits_workfn(struct work_struct *work)
>  {
> +	struct loop_device *lo =
> +		container_of(work, struct loop_device, clear_limits_work);
>  	struct queue_limits lim = queue_limits_start_update(lo->lo_queue);
> -
> -	if (mode & FALLOC_FL_ZERO_RANGE)
> -		lim.max_write_zeroes_sectors = 0;
> -
> -	if (mode & FALLOC_FL_PUNCH_HOLE) {
> -		lim.max_hw_discard_sectors = 0;
> -		lim.discard_granularity = 0;
> -	}
> +	unsigned int memflags;
> +	int mode = 0;
>  
>  	/*
> -	 * XXX: this updates the queue limits without freezing the queue, which
> -	 * is against the locking protocol and dangerous.  But we can't just
> -	 * freeze the queue as we're inside the ->queue_rq method here.  So this
> -	 * should move out into a workqueue unless we get the file operations to
> -	 * advertise if they support specific fallocate operations.
> +	 * Commit the unmodified limits if the device was rebound since
> +	 * the work item was scheduled.  The freeze does not exclude a
> +	 * rebind through loop_change_fd(), which freezes the queue
> +	 * itself, so hold clear_limits_lock over the generation check
> +	 * and the commit: loop_assign_backing_file() bumps rebind_gen
> +	 * under the same mutex, so a rebind either precedes the check
> +	 * or follows the commit, and a stale clear cannot hit the new
> +	 * backing file.  The other rebinding paths, loop_configure()
> +	 * and __loop_clr_fd(), bump rebind_gen under the same mutex
> +	 * without freezing the queue; the generation check detects
> +	 * them as well.
>  	 */
> +	memflags = blk_mq_freeze_queue(lo->lo_queue);
> +	mutex_lock(&lo->clear_limits_lock);
> +	if (lo->clear_limits_gen == lo->rebind_gen) {
> +		mode = lo->clear_limits_mode;
> +		lo->clear_limits_mode = 0;
> +
> +		if (mode & FALLOC_FL_ZERO_RANGE)
> +			lim.max_write_zeroes_sectors = 0;
> +
> +		if (mode & FALLOC_FL_PUNCH_HOLE) {
> +			lim.max_hw_discard_sectors = 0;
> +			lim.discard_granularity = 0;
> +		}
> +	}
>  	queue_limits_commit_update(lo->lo_queue, &lim);
> +	mutex_unlock(&lo->clear_limits_lock);
> +	blk_mq_unfreeze_queue(lo->lo_queue, memflags);
> +}
> +
> +static void loop_clear_limits(struct loop_device *lo, int mode)
> +{
> +	mutex_lock(&lo->clear_limits_lock);
> +	lo->clear_limits_gen = lo->rebind_gen;
> +	lo->clear_limits_mode |= mode;
> +	mutex_unlock(&lo->clear_limits_lock);
> +	schedule_work(&lo->clear_limits_work);
>  }
>  
>  static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
> @@ -518,6 +550,10 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
>  static void loop_assign_backing_file(struct loop_device *lo, struct file *file)
>  {
>  	lo->lo_backing_file = file;
> +	mutex_lock(&lo->clear_limits_lock);
> +	lo->rebind_gen++;
> +	lo->clear_limits_mode = 0;
> +	mutex_unlock(&lo->clear_limits_lock);
>  	lo->old_gfp_mask = mapping_gfp_mask(file->f_mapping);
>  	mapping_set_gfp_mask(file->f_mapping,
>  			lo->old_gfp_mask & ~(__GFP_IO | __GFP_FS));
> @@ -1148,6 +1184,15 @@ static void __loop_clr_fd(struct loop_device *lo)
>  	lo->lo_backing_file = NULL;
>  	spin_unlock_irq(&lo->lo_lock);
>  
> +	/*
> +	 * Invalidate any pending clear that was scheduled against the old
> +	 * backing file, like loop_assign_backing_file() does on rebind.
> +	 */
> +	mutex_lock(&lo->clear_limits_lock);
> +	lo->rebind_gen++;
> +	lo->clear_limits_mode = 0;
> +	mutex_unlock(&lo->clear_limits_lock);
> +
>  	lo->lo_device = NULL;
>  	lo->lo_offset = 0;
>  	lo->lo_sizelimit = 0;
> @@ -1783,7 +1828,9 @@ static void lo_free_disk(struct gendisk *disk)
>  		destroy_workqueue(lo->workqueue);
>  	loop_free_idle_workers(lo, true);
>  	timer_shutdown_sync(&lo->timer);
> +	cancel_work_sync(&lo->clear_limits_work);
>  	mutex_destroy(&lo->lo_mutex);
> +	mutex_destroy(&lo->clear_limits_lock);
>  	kfree(lo);
>  }
>  
> @@ -2102,6 +2149,8 @@ static int loop_add(int i)
>  	spin_lock_init(&lo->lo_lock);
>  	spin_lock_init(&lo->lo_work_lock);
>  	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
> +	INIT_WORK(&lo->clear_limits_work, loop_clear_limits_workfn);
> +	mutex_init(&lo->clear_limits_lock);
>  	INIT_LIST_HEAD(&lo->rootcg_cmd_list);
>  	disk->major		= LOOP_MAJOR;
>  	disk->first_minor	= i << part_shift;
> @@ -2140,6 +2189,12 @@ static int loop_add(int i)
>  
>  static void loop_remove(struct loop_device *lo)
>  {
> +	/*
> +	 * Cancel early: the queue may already be in RCU-delayed freeing
> +	 * by the time lo_free_disk() cancels the work item.
> +	 */
> +	cancel_work_sync(&lo->clear_limits_work);
> +
>  	/* Make this loop device unreachable from pathname. */
>  	del_gendisk(lo->lo_disk);
>  	blk_mq_free_tag_set(&lo->tag_set);