Re: [PATCH v3 7/7] zram: Use local lock to protect per-CPU data

[Yu Zhao <yuzhao@google.com>]

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 ***