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ublk: drop the device reference outside ublk_ctl_mutex in DEL_DEV

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ublk_ctrl_del_dev() drops its reference to the device while holding the
global ublk_ctl_mutex. When this is the last reference, ublk_cdev_rel()
frees the tag set, and blk_mq_free_tag_set() waits for the pending SRCU
callbacks of the tag set with srcu_barrier(). Every DEL_DEV thus waits
for an SRCU grace period with the mutex held, and deleting devices from
several threads serializes on it.

The release path doesn't need ublk_ctl_mutex. The device number is freed
under ublk_idr_lock, and the last reference is already dropped without
the mutex when the ublk server still has the char device open at
DEL_DEV time, from ublk_ch_release_work_fn().

Drop the reference after unlocking ublk_ctl_mutex.

ublk null target, 4000 single-queue devices, STOP_DEV + DEL_DEV issued
from N threads, 16 vCPU KVM guest:

  threads     before      after
        1     77.6 s     77.6 s
        4     20.1 s     19.4 s
        8     19.3 s      9.0 s
       16     19.2 s      4.1 s

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20261001-bug-ublk-del-dev-put-unlocked-v1-1-18e9bb9be727@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Qiliang Yuan 2026-10-01 11:10:47 +08:00 committed by Jens Axboe
parent 2b8ee88f18
commit 812485c8ff
1 changed files with 7 additions and 2 deletions
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9
drivers/block/ublk_drv.c
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@ -4901,10 +4901,15 @@ static int ublk_ctrl_del_dev(struct ublk_device **p_ub, bool wait)
set_bit(UB_STATE_DELETED, &ub->state);
}
/* Mark the reference as consumed */
mutex_unlock(&ublk_ctl_mutex);
/*
* Drop the reference outside ublk_ctl_mutex: if it is the last one,
* the release frees the tag set, which waits for an SRCU grace period,
* and holding the mutex would serialize concurrent deletions on it.
*/
*p_ub = NULL;
ublk_put_device(ub);
mutex_unlock(&ublk_ctl_mutex);
/*
* Wait until the idr is removed, then it can be reused after