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SubjectRe: [PATCH v3 7/7] zram: Use local lock to protect per-CPU data
On Wed, May 27, 2020 at 10:11:19PM +0200, Sebastian Andrzej Siewior wrote:
> From: Mike Galbraith <umgwanakikbuti@gmail.com>
>
> The zcomp driver uses per-CPU compression. The per-CPU data pointer is
> acquired with get_cpu_ptr() which implicitly disables preemption.
> It allocates memory inside the preempt disabled region which conflicts
> with the PREEMPT_RT semantics.
>
> Replace the implicit preemption control with an explicit local lock.
> This allows RT kernels to substitute it with a real per CPU lock, which
> serializes the access but keeps the code section preemptible. On non RT
> kernels this maps to preempt_disable() as before, i.e. no functional
> change.

Hi,

This change seems to have introduced a potential deadlock. Can you
please take a look?

Thank you.

[ 40.030778] ======================================================
[ 40.037706] WARNING: possible circular locking dependency detected
[ 40.044637] 5.9.0-74216-g5c9472ed6825 #1 Tainted: G W
[ 40.051759] ------------------------------------------------------
[ 40.058685] swapon/586 is trying to acquire lock:
[ 40.063950] ffffe8ffffc0ee60 (&zstrm->lock){+.+.}-{2:2}, at: local_lock_acquire+0x5/0x70 [zram]
[ 40.073739]
[ 40.073739] but task is already holding lock:
[ 40.080277] ffff888101a1f438 (&zspage->lock){.+.+}-{2:2}, at: zs_map_object+0x73/0x28d
[ 40.089182]
[ 40.089182] which lock already depends on the new lock.
[ 40.089182]
[ 40.098344]
[ 40.098344] the existing dependency chain (in reverse order) is:
[ 40.106715]
[ 40.106715] -> #1 (&zspage->lock){.+.+}-{2:2}:
[ 40.113386] lock_acquire+0x1cd/0x2c3
[ 40.118083] _raw_read_lock+0x44/0x78
[ 40.122781] zs_map_object+0x73/0x28d
[ 40.127479] zram_bvec_rw+0x42e/0x75d [zram]
[ 40.132855] zram_submit_bio+0x1fc/0x2d7 [zram]
[ 40.138526] submit_bio_noacct+0x11b/0x372
[ 40.143709] submit_bio+0xfd/0x1b5
[ 40.148113] __block_write_full_page+0x302/0x56f
[ 40.153877] __writepage+0x1e/0x74
[ 40.158281] write_cache_pages+0x404/0x59a
[ 40.163461] generic_writepages+0x53/0x82
[ 40.168545] do_writepages+0x33/0x74
[ 40.173145] __filemap_fdatawrite_range+0x91/0xac
[ 40.179005] file_write_and_wait_range+0x39/0x87
[ 40.184769] blkdev_fsync+0x19/0x3e
[ 40.189272] do_fsync+0x39/0x5c
[ 40.193384] __x64_sys_fsync+0x13/0x17
[ 40.198178] do_syscall_64+0x37/0x45
[ 40.202776] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 40.209022]
[ 40.209022] -> #0 (&zstrm->lock){+.+.}-{2:2}:
[ 40.215589] validate_chain+0x1966/0x21a8
[ 40.220673] __lock_acquire+0x941/0xbba
[ 40.225552] lock_acquire+0x1cd/0x2c3
[ 40.230250] local_lock_acquire+0x21/0x70 [zram]
[ 40.236015] zcomp_stream_get+0x33/0x4d [zram]
[ 40.241585] zram_bvec_rw+0x476/0x75d [zram]
[ 40.246963] zram_rw_page+0xd8/0x17c [zram]
[ 40.252240] bdev_read_page+0x7a/0x9d
[ 40.256933] do_mpage_readpage+0x6b2/0x860
[ 40.262101] mpage_readahead+0x136/0x245
[ 40.267089] read_pages+0x60/0x1f9
[ 40.271492] page_cache_ra_unbounded+0x211/0x27b
[ 40.277251] generic_file_buffered_read+0x188/0xd4d
[ 40.283296] new_sync_read+0x10c/0x143
[ 40.288088] vfs_read+0xf4/0x1a5
[ 40.292285] ksys_read+0x73/0xd3
[ 40.296483] do_syscall_64+0x37/0x45
[ 40.301072] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 40.307319]
[ 40.307319] other info that might help us debug this:
[ 40.307319]
[ 40.316285] Possible unsafe locking scenario:
[ 40.316285]
[ 40.322907] CPU0 CPU1
[ 40.327972] ---- ----
[ 40.333041] lock(&zspage->lock);
[ 40.336874] lock(&zstrm->lock);
[ 40.343424] lock(&zspage->lock);
[ 40.350071] lock(&zstrm->lock);
[ 40.353803]
[ 40.353803] *** DEADLOCK ***

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