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Message-ID: <20200723180902.GV3673@sequoia>
Date: Thu, 23 Jul 2020 13:09:02 -0500
From: Tyler Hicks <tyhicks@...ux.microsoft.com>
To: Pavel Tatashin <pasha.tatashin@...een.com>
Cc: axboe@...nel.dk, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] loop: scale loop device by introducing per device
lock
On 2020-07-17 16:53:22, Pavel Tatashin wrote:
> Currently, loop device has only one global lock:
> loop_ctl_mutex.
>
> This becomes hot in scenarios where many loop devices are used.
>
> Scale it by introducing per-device lock: lo_mutex that proctests
> field in struct loop_device. Keep loop_ctl_mutex to protect global
s/proctests field/protects the fields/
> data such as loop_index_idr, loop_lookup, loop_add.
>
> Lock ordering: loop_ctl_mutex > lo_mutex.
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> ---
> drivers/block/loop.c | 86 ++++++++++++++++++++++++--------------------
> drivers/block/loop.h | 1 +
> 2 files changed, 48 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 475e1a738560..056af3bca6c2 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -706,7 +706,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> int error;
> bool partscan;
>
> - error = mutex_lock_killable(&loop_ctl_mutex);
> + error = mutex_lock_killable(&lo->lo_mutex);
> if (error)
> return error;
> error = -ENXIO;
> @@ -745,9 +745,9 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> loop_update_dio(lo);
> blk_mq_unfreeze_queue(lo->lo_queue);
> partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> /*
> - * We must drop file reference outside of loop_ctl_mutex as dropping
> + * We must drop file reference outside of lo_mutex as dropping
> * the file ref can take bd_mutex which creates circular locking
> * dependency.
> */
> @@ -757,7 +757,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> return 0;
>
> out_err:
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> if (file)
> fput(file);
> return error;
> @@ -1096,7 +1096,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> }
> }
>
> - error = mutex_lock_killable(&loop_ctl_mutex);
> + error = mutex_lock_killable(&lo->lo_mutex);
> if (error)
> goto out_bdev;
>
> @@ -1176,7 +1176,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> * put /dev/loopXX inode. Later in __loop_clr_fd() we bdput(bdev).
> */
> bdgrab(bdev);
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> if (partscan)
> loop_reread_partitions(lo, bdev);
> if (claimed_bdev)
> @@ -1184,7 +1184,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> return 0;
>
> out_unlock:
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> out_bdev:
> if (claimed_bdev)
> bd_abort_claiming(bdev, claimed_bdev, loop_configure);
> @@ -1205,7 +1205,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> bool partscan = false;
> int lo_number;
>
> - mutex_lock(&loop_ctl_mutex);
> + mutex_lock(&lo->lo_mutex);
> if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
> err = -ENXIO;
> goto out_unlock;
> @@ -1259,7 +1259,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> lo_number = lo->lo_number;
> loop_unprepare_queue(lo);
> out_unlock:
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> if (partscan) {
> /*
> * bd_mutex has been held already in release path, so don't
> @@ -1290,18 +1290,18 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> * protects us from all the other places trying to change the 'lo'
> * device.
> */
> - mutex_lock(&loop_ctl_mutex);
> + mutex_lock(&lo->lo_mutex);
> lo->lo_flags = 0;
> if (!part_shift)
> lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
> lo->lo_state = Lo_unbound;
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
>
> /*
> - * Need not hold loop_ctl_mutex to fput backing file.
> - * Calling fput holding loop_ctl_mutex triggers a circular
> + * Need not hold lo_mutex to fput backing file.
> + * Calling fput holding lo_mutex triggers a circular
> * lock dependency possibility warning as fput can take
> - * bd_mutex which is usually taken before loop_ctl_mutex.
> + * bd_mutex which is usually taken before lo_mutex.
> */
> if (filp)
> fput(filp);
> @@ -1312,11 +1312,11 @@ static int loop_clr_fd(struct loop_device *lo)
> {
> int err;
>
> - err = mutex_lock_killable(&loop_ctl_mutex);
> + err = mutex_lock_killable(&lo->lo_mutex);
> if (err)
> return err;
> if (lo->lo_state != Lo_bound) {
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> return -ENXIO;
> }
> /*
> @@ -1331,11 +1331,11 @@ static int loop_clr_fd(struct loop_device *lo)
> */
> if (atomic_read(&lo->lo_refcnt) > 1) {
> lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> return 0;
> }
> lo->lo_state = Lo_rundown;
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
>
> return __loop_clr_fd(lo, false);
> }
> @@ -1350,7 +1350,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
> bool partscan = false;
> bool size_changed = false;
>
> - err = mutex_lock_killable(&loop_ctl_mutex);
> + err = mutex_lock_killable(&lo->lo_mutex);
> if (err)
> return err;
> if (lo->lo_encrypt_key_size &&
> @@ -1417,7 +1417,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
> partscan = true;
> }
> out_unlock:
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> if (partscan)
> loop_reread_partitions(lo, bdev);
>
> @@ -1431,11 +1431,11 @@ loop_get_status(struct loop_device *lo, struct loop_info64 *info)
> struct kstat stat;
> int ret;
>
> - ret = mutex_lock_killable(&loop_ctl_mutex);
> + ret = mutex_lock_killable(&lo->lo_mutex);
> if (ret)
> return ret;
> if (lo->lo_state != Lo_bound) {
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> return -ENXIO;
> }
>
> @@ -1454,10 +1454,10 @@ loop_get_status(struct loop_device *lo, struct loop_info64 *info)
> lo->lo_encrypt_key_size);
> }
>
> - /* Drop loop_ctl_mutex while we call into the filesystem. */
> + /* Drop lo_mutex while we call into the filesystem. */
> path = lo->lo_backing_file->f_path;
> path_get(&path);
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> ret = vfs_getattr(&path, &stat, STATX_INO, AT_STATX_SYNC_AS_STAT);
> if (!ret) {
> info->lo_device = huge_encode_dev(stat.dev);
> @@ -1643,7 +1643,7 @@ static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,
> {
> int err;
>
> - err = mutex_lock_killable(&loop_ctl_mutex);
> + err = mutex_lock_killable(&lo->lo_mutex);
> if (err)
> return err;
> switch (cmd) {
> @@ -1659,7 +1659,7 @@ static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,
> default:
> err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
> }
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> return err;
> }
>
> @@ -1890,22 +1890,23 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
> return err;
> lo = bdev->bd_disk->private_data;
> if (!lo) {
> - err = -ENXIO;
> - goto out;
> + mutex_unlock(&loop_ctl_mutex);
> + return -ENXIO;
> }
> -
> - atomic_inc(&lo->lo_refcnt);
> -out:
> + err = mutex_lock_killable(&lo->lo_mutex);
> mutex_unlock(&loop_ctl_mutex);
I don't see a possibility for deadlock but it bothers me a little that
we're not unlocking in the reverse locking order here, as we do in
loop_control_ioctl(). There should be no perf impact if we move the
mutex_unlock(&loop_ctl_mutex) after mutex_unlock(&lo->lo_mutex).
> - return err;
> + if (err)
> + return err;
> + atomic_inc(&lo->lo_refcnt);
> + mutex_unlock(&lo->lo_mutex);
> + return 0;
> }
>
> static void lo_release(struct gendisk *disk, fmode_t mode)
> {
> - struct loop_device *lo;
> + struct loop_device *lo = disk->private_data;
>
> - mutex_lock(&loop_ctl_mutex);
> - lo = disk->private_data;
> + mutex_lock(&lo->lo_mutex);
> if (atomic_dec_return(&lo->lo_refcnt))
> goto out_unlock;
>
> @@ -1913,7 +1914,7 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
> if (lo->lo_state != Lo_bound)
> goto out_unlock;
> lo->lo_state = Lo_rundown;
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> /*
> * In autoclear mode, stop the loop thread
> * and remove configuration after last close.
> @@ -1930,7 +1931,7 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
> }
>
> out_unlock:
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> }
>
> static const struct block_device_operations lo_fops = {
> @@ -1969,10 +1970,10 @@ static int unregister_transfer_cb(int id, void *ptr, void *data)
> struct loop_device *lo = ptr;
> struct loop_func_table *xfer = data;
>
> - mutex_lock(&loop_ctl_mutex);
> + mutex_lock(&lo->lo_mutex);
> if (lo->lo_encryption == xfer)
> loop_release_xfer(lo);
> - mutex_unlock(&loop_ctl_mutex);
> + mutex_unlock(&lo->lo_mutex);
> return 0;
> }
>
> @@ -2157,6 +2158,7 @@ static int loop_add(struct loop_device **l, int i)
> disk->flags |= GENHD_FL_NO_PART_SCAN;
> disk->flags |= GENHD_FL_EXT_DEVT;
> atomic_set(&lo->lo_refcnt, 0);
> + mutex_init(&lo->lo_mutex);
We need a corresponding call to mutex_destroy() in loop_remove().
> lo->lo_number = i;
> spin_lock_init(&lo->lo_lock);
> disk->major = LOOP_MAJOR;
> @@ -2272,15 +2274,21 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
> ret = loop_lookup(&lo, parm);
> if (ret < 0)
> break;
> + ret = mutex_lock_killable(&lo->lo_mutex);
> + if (ret)
> + break;
> if (lo->lo_state != Lo_unbound) {
> ret = -EBUSY;
> + mutex_unlock(&lo->lo_mutex);
> break;
> }
> if (atomic_read(&lo->lo_refcnt) > 0) {
> ret = -EBUSY;
> + mutex_unlock(&lo->lo_mutex);
> break;
> }
> lo->lo_disk->private_data = NULL;
> + mutex_unlock(&lo->lo_mutex);
> idr_remove(&loop_index_idr, lo->lo_number);
> loop_remove(lo);
> break;
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index af75a5ee4094..a3c04f310672 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -62,6 +62,7 @@ struct loop_device {
> struct request_queue *lo_queue;
> struct blk_mq_tag_set tag_set;
> struct gendisk *lo_disk;
There's an instance, which is not in this patch's context, of accessing
lo_disk that needs lo_mutex protection. In loop_probe(), we call
get_disk_and_module(lo->lo_disk) and we need to lock and unlock lo_mutex
around that call.
Tyler
> + struct mutex lo_mutex;
> };
>
> struct loop_cmd {
> --
> 2.25.1
>
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