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blk-cgroup: fix circular locking dependency in blkg_conf_prep - #1124

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blk-cgroup: fix circular locking dependency in blkg_conf_prep#1124
blktests-ci-kpd[bot] wants to merge 1 commit into
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@blktests-ci-kpd blktests-ci-kpd Bot commented Aug 11, 2026

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Pull request for series with
subject: blk-cgroup: fix circular locking dependency in blkg_conf_prep
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1144320

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Upstream branch: d58772d
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: f5bbbfe
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from e15a2ce to 30451ea Compare August 12, 2026 13:28
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Upstream branch: 3d6d817
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from 30451ea to 1754e5e Compare August 12, 2026 19:18
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Upstream branch: 3aa1dca
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from 1754e5e to 328ef59 Compare August 13, 2026 19:20
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Upstream branch: fd923b3
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: 8d3ae59
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: bd5f485
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: 66498c7
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: 502d457
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: cf72cbb
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: cee9395
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: 89a3129
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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Upstream branch: bc35965
series: https://patchwork.kernel.org/project/linux-block/list/?series=1144320
version: 1

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blktests-ci-kpd Bot commented Sep 7, 2026

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Upstream branch: df29080
series: https://patchwork.kernel.org/series/1144320/
version: 1

Pull request is NOT updated. Failed to apply https://patchwork.kernel.org/series/1144320/
error message:

Cmd('git') failed due to: exit code(128)
  cmdline: git am --3way
  stdout: 'Patch is empty.
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To record the empty patch as an empty commit, run "git am --allow-empty".
To restore the original branch and stop patching, run "git am --abort".'

conflict:

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Upstream branch: 28924df
series: https://patchwork.kernel.org/series/1144320/
version: 1

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Upstream branch: 28924df
series: https://patchwork.kernel.org/series/1144320/
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from 721b883 to 170733a Compare September 8, 2026 16:33
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blktests-ci-kpd Bot commented Sep 9, 2026

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Upstream branch: 893e117
series: https://patchwork.kernel.org/series/1144320/
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from 170733a to 59948df Compare September 9, 2026 08:28
Sudarsan Mahendran kernel-patches-daemon
blk-cgroup: fix circular locking dependency in blkg_conf_prep
2eda3f1 
In blkg_conf_prep, we may do IO when allocating while
holding blkcg_mutex, however this presents a deadlock
scenario since we may grab sysfs_lock while IO is
suspended in elevator_exit, and an existing chain
exists from sysfs_mutex -> blkcg_mutex.

We can fix this by performing only GFP_NOIO allocations within
blkg_conf_prep when blkcg_mutex is held.
This patch fixes our reproducer.

Full lockdep splat:

[ 3795.003422] ======================================================
[ 3795.009595] WARNING: possible circular locking dependency detected
[ 3795.015773] 6.18.20-dbg-DEV #1 Tainted: G                 N
[ 3795.021432] ------------------------------------------------------
[ 3795.027604] kpython3/435194 is trying to acquire lock:
[ 3795.032744] ffffffffa36959b8 (fs_reclaim){+.+.}-{0:0}, at: kmem_cache_alloc_noprof+0x49/0x5c0
[ 3795.041260]
[ 3795.041260] but task is already holding lock:
[ 3795.047086] ffff98c8a06477b0 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0x71/0x5e0
[ 3795.055257]
[ 3795.055257] which lock already depends on the new lock.
[ 3795.055257]
[ 3795.063422]
[ 3795.063422] the existing dependency chain (in reverse order) is:
[ 3795.070893]
[ 3795.070893] -> #4 (&q->blkcg_mutex){+.+.}-{4:4}:
[ 3795.076986]        __mutex_lock+0x92/0xb70
[ 3795.081085]        blkcg_deactivate_policy+0x7f/0x230
[ 3795.086137]        bfq_exit_queue+0xe1/0x110
[ 3795.090401]        blk_mq_exit_sched+0xd1/0x170
[ 3795.094935]        elevator_exit+0x50/0x70
(...)
[ 3795.135019]
[ 3795.135019] -> #3 (&eq->sysfs_lock){+.+.}-{4:4}:
[ 3795.141110]        __mutex_lock+0x92/0xb70
[ 3795.145211]        elevator_exit+0x45/0x70
[ 3795.149308]        elevator_switch+0x98/0x290
[ 3795.153660]        elevator_change+0x109/0x190
[ 3795.158106]        elv_iosched_store+0x198/0x240
(...)
[ 3795.185276]
[ 3795.185276] -> #2 (&q->elevator_lock){+.+.}-{4:4}:
[ 3795.191541]        __mutex_lock+0x92/0xb70
[ 3795.195642]        elevator_change+0xb6/0x190
[ 3795.200001]        elv_iosched_store+0x198/0x240
(...)
[ 3795.227179]
[ 3795.227179] -> #1 (&q->q_usage_counter(io)#19){++++}-{0:0}:
[ 3795.234224]        blk_alloc_queue+0x33e/0x370
(...)
[ 3795.266621]
[ 3795.266621] -> #0 (fs_reclaim){+.+.}-{0:0}:
[ 3795.272280]        __lock_acquire+0x157c/0x2980
[ 3795.276813]        lock_acquire+0xd3/0x2a0
[ 3795.280904]        fs_reclaim_acquire+0x55/0xc0
[ 3795.285435]        kmem_cache_alloc_noprof+0x49/0x5c0
[ 3795.290481]        __radix_tree_preload+0xef/0x250
[ 3795.295273]        blkg_conf_prep+0x2aa/0x5e0
[ 3795.299633]        tg_set_conf+0xa5/0x1a0
(...)
[ 3795.330808]
[ 3795.330808] other info that might help us debug this:
[ 3795.330808]
[ 3795.338806] Chain exists of:
[ 3795.338806]   fs_reclaim --> &eq->sysfs_lock --> &q->blkcg_mutex
[ 3795.338806]
[ 3795.349146]  Possible unsafe locking scenario:
[ 3795.349146]
[ 3795.355057]        CPU0                    CPU1
[ 3795.359590]        ----                    ----
[ 3795.364121]   lock(&q->blkcg_mutex);
[ 3795.367691]                                lock(&eq->sysfs_lock);
[ 3795.373776]                                lock(&q->blkcg_mutex);
[ 3795.379862]   lock(fs_reclaim);
[ 3795.383006]
[ 3795.383006]  *** DEADLOCK ***

Reviewed-by: Khazhy Kumykov <khazhy@google.com>
Signed-off-by: Sudarsan Mahendran <sudarsanm@google.com>
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blktests-ci-kpd Bot commented Sep 10, 2026

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Upstream branch: 50d05c7
series: https://patchwork.kernel.org/series/1144320/
version: 1

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blktests-ci-kpd Bot force-pushed the series/1144320=>linus-master branch from 59948df to 2eda3f1 Compare September 10, 2026 15:59
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blktests-ci-kpd Bot commented Sep 10, 2026

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At least one diff in series https://patchwork.kernel.org/series/1144320/ irrelevant now for [{'archived': False, 'project': 241}] search patterns

@blktests-ci-kpd blktests-ci-kpd Bot closed this Sep 10, 2026
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