Hi
I'd like to ask - why not use dm_bufio_trylock from an interrupt context?
I would just add a new function "dm_bufio_get_trylock" that is equivalent
to "dm_bufio_get", except that it uses dm_bufio_trylock - and if it fails,
fallback to process context.
I think using dm_bufio_trylock would be less hacky than introducing a
new dm_bufio flag that changes mutex to spinlock.
Mikulas
On Tue, 26 Jul 2022, Mike Snitzer wrote:
> From: Nathan Huckleberry <nhuck@xxxxxxxxxx>
>
> Add an optional flag that ensures dm_bufio_client does not sleep
> (primary focus is to service dm_bufio_get without sleeping). This
> allows the dm-bufio cache to be queried from interrupt context.
>
> To ensure that dm-bufio does not sleep, dm-bufio must use a spinlock
> instead of a mutex. Additionally, to avoid deadlocks, special care
> must be taken so that dm-bufio does not sleep while holding the
> spinlock.
>
> But again: the scope of this no_sleep is initially confined to
> dm_bufio_get, so __alloc_buffer_wait_no_callback is _not_ changed to
> avoid sleeping because __bufio_new avoids allocation for NF_GET.
>
> Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> ---
> drivers/md/dm-bufio.c | 22 +++++++++++++++++++---
> include/linux/dm-bufio.h | 5 +++++
> 2 files changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> index ad5603eb12e3..486179d1831c 100644
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -81,6 +81,8 @@
> */
> struct dm_bufio_client {
> struct mutex lock;
> + spinlock_t spinlock;
> + unsigned long spinlock_flags;
>
> struct list_head lru[LIST_SIZE];
> unsigned long n_buffers[LIST_SIZE];
> @@ -90,6 +92,7 @@ struct dm_bufio_client {
> s8 sectors_per_block_bits;
> void (*alloc_callback)(struct dm_buffer *);
> void (*write_callback)(struct dm_buffer *);
> + bool no_sleep;
>
> struct kmem_cache *slab_buffer;
> struct kmem_cache *slab_cache;
> @@ -167,17 +170,26 @@ struct dm_buffer {
>
> static void dm_bufio_lock(struct dm_bufio_client *c)
> {
> - mutex_lock_nested(&c->lock, dm_bufio_in_request());
> + if (c->no_sleep)
> + spin_lock_irqsave_nested(&c->spinlock, c->spinlock_flags, dm_bufio_in_request());
> + else
> + mutex_lock_nested(&c->lock, dm_bufio_in_request());
> }
>
> static int dm_bufio_trylock(struct dm_bufio_client *c)
> {
> - return mutex_trylock(&c->lock);
> + if (c->no_sleep)
> + return spin_trylock_irqsave(&c->spinlock, c->spinlock_flags);
> + else
> + return mutex_trylock(&c->lock);
> }
>
> static void dm_bufio_unlock(struct dm_bufio_client *c)
> {
> - mutex_unlock(&c->lock);
> + if (c->no_sleep)
> + spin_unlock_irqrestore(&c->spinlock, c->spinlock_flags);
> + else
> + mutex_unlock(&c->lock);
> }
>
> /*----------------------------------------------------------------*/
> @@ -1748,12 +1760,16 @@ struct dm_bufio_client *dm_bufio_client_create(struct block_device *bdev, unsign
> c->alloc_callback = alloc_callback;
> c->write_callback = write_callback;
>
> + if (flags & DM_BUFIO_CLIENT_NO_SLEEP)
> + c->no_sleep = true;
> +
> for (i = 0; i < LIST_SIZE; i++) {
> INIT_LIST_HEAD(&c->lru[i]);
> c->n_buffers[i] = 0;
> }
>
> mutex_init(&c->lock);
> + spin_lock_init(&c->spinlock);
> INIT_LIST_HEAD(&c->reserved_buffers);
> c->need_reserved_buffers = reserved_buffers;
>
> diff --git a/include/linux/dm-bufio.h b/include/linux/dm-bufio.h
> index e21480715255..15d9e15ca830 100644
> --- a/include/linux/dm-bufio.h
> +++ b/include/linux/dm-bufio.h
> @@ -17,6 +17,11 @@
> struct dm_bufio_client;
> struct dm_buffer;
>
> +/*
> + * Flags for dm_bufio_client_create
> + */
> +#define DM_BUFIO_CLIENT_NO_SLEEP 0x1
> +
> /*
> * Create a buffered IO cache on a given device
> */
> --
> 2.32.1 (Apple Git-133)
>
> --
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>
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