Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

[Stephen Boyd]

On 08/01/14 03:27, Prarit Bhargava wrote:
>
> Can you send me the test and the trace of the deadlock?  I'm not creating it with:
>

This was with conservative as the default, and switching to ondemand

# cd /sys/devices/system/cpu/cpu2/cpufreq
# ls
affected_cpus                  scaling_available_governors
conservative                   scaling_cur_freq
cpuinfo_cur_freq               scaling_driver
cpuinfo_max_freq               scaling_governor
cpuinfo_min_freq               scaling_max_freq
cpuinfo_transition_latency     scaling_min_freq
related_cpus                   scaling_setspeed
scaling_available_frequencies  stats
# cat conservative/down_threshold
20
# echo ondemand > scaling_governor

 ======================================================
 [ INFO: possible circular locking dependency detected ]
 3.16.0-rc3-00039-ge1e38f124d87 #47 Not tainted
 -------------------------------------------------------
 sh/75 is trying to acquire lock:
  (s_active#9){++++..}, at: [<c0358a94>] kernfs_remove_by_name_ns+0x3c/0x84

 but task is already holding lock:
  (&policy->rwsem){+++++.}, at: [<c05ab1f0>] store+0x68/0xb8

 which lock already depends on the new lock.


 the existing dependency chain (in reverse order) is:

-> #1 (&policy->rwsem){+++++.}:
        [<c0359234>] kernfs_fop_open+0x138/0x298
        [<c02fa3f4>] do_dentry_open.isra.12+0x1b0/0x2f0
        [<c02fa604>] finish_open+0x20/0x38
        [<c0308d34>] do_last.isra.37+0x5ac/0xb68
        [<c03093a4>] path_openat+0xb4/0x5d8
        [<c0309bcc>] do_filp_open+0x2c/0x80
        [<c02fb558>] do_sys_open+0x10c/0x1c8
        [<c020f0a0>] ret_fast_syscall+0x0/0x48

-> #0 (s_active#9){++++..}:
        [<c0357d18>] __kernfs_remove+0x250/0x300
        [<c0358a94>] kernfs_remove_by_name_ns+0x3c/0x84
        [<c035aa78>] remove_files+0x34/0x78
        [<c035aee0>] sysfs_remove_group+0x40/0x98
        [<c05b0560>] cpufreq_governor_dbs+0x4c0/0x6ec
        [<c05abebc>] __cpufreq_governor+0x118/0x200
        [<c05ac0fc>] cpufreq_set_policy+0x158/0x2ac
        [<c05ad5e4>] store_scaling_governor+0x6c/0x94
        [<c05ab210>] store+0x88/0xb8
        [<c035a00c>] sysfs_kf_write+0x4c/0x50
        [<c03594d4>] kernfs_fop_write+0xc0/0x180
        [<c02fc5c8>] vfs_write+0xa0/0x1a8
        [<c02fc9d4>] SyS_write+0x40/0x8c
        [<c020f0a0>] ret_fast_syscall+0x0/0x48

 other info that might help us debug this:

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(&policy->rwsem);
                                lock(s_active#9);
                                lock(&policy->rwsem);
   lock(s_active#9);

  *** DEADLOCK ***

 6 locks held by sh/75:
  #0:  (sb_writers#4){.+.+..}, at: [<c02fc6a8>] vfs_write+0x180/0x1a8
  #1:  (&of->mutex){+.+...}, at: [<c0359498>] kernfs_fop_write+0x84/0x180
  #2:  (s_active#10){.+.+..}, at: [<c03594a0>] kernfs_fop_write+0x8c/0x180
  #3:  (cpu_hotplug.lock){++++++}, at: [<c0221ef8>] get_online_cpus+0x38/0x9c
  #4:  (cpufreq_rwsem){.+.+.+}, at: [<c05ab1d8>] store+0x50/0xb8
  #5:  (&policy->rwsem){+++++.}, at: [<c05ab1f0>] store+0x68/0xb8

 stack backtrace:
 CPU: 0 PID: 75 Comm: sh Not tainted 3.16.0-rc3-00039-ge1e38f124d87 #47
 [<c0214de8>] (unwind_backtrace) from [<c02123f8>] (show_stack+0x10/0x14)
 [<c02123f8>] (show_stack) from [<c0709e5c>] (dump_stack+0x70/0xbc)
 [<c0709e5c>] (dump_stack) from [<c070722c>] (print_circular_bug+0x280/0x2d4)
 [<c070722c>] (print_circular_bug) from [<c02629cc>] (__lock_acquire+0x18d0/0x1abc)
 [<c02629cc>] (__lock_acquire) from [<c026310c>] (lock_acquire+0x9c/0x138)
 [<c026310c>] (lock_acquire) from [<c0357d18>] (__kernfs_remove+0x250/0x300)
 [<c0357d18>] (__kernfs_remove) from [<c0358a94>] (kernfs_remove_by_name_ns+0x3c/0x84)
 [<c0358a94>] (kernfs_remove_by_name_ns) from [<c035aa78>] (remove_files+0x34/0x78)
 [<c035aa78>] (remove_files) from [<c035aee0>] (sysfs_remove_group+0x40/0x98)
 [<c035aee0>] (sysfs_remove_group) from [<c05b0560>] (cpufreq_governor_dbs+0x4c0/0x6ec)
 [<c05b0560>] (cpufreq_governor_dbs) from [<c05abebc>] (__cpufreq_governor+0x118/0x200)
 [<c05abebc>] (__cpufreq_governor) from [<c05ac0fc>] (cpufreq_set_policy+0x158/0x2ac)
 [<c05ac0fc>] (cpufreq_set_policy) from [<c05ad5e4>] (store_scaling_governor+0x6c/0x94)
 [<c05ad5e4>] (store_scaling_governor) from [<c05ab210>] (store+0x88/0xb8)
 [<c05ab210>] (store) from [<c035a00c>] (sysfs_kf_write+0x4c/0x50)
 [<c035a00c>] (sysfs_kf_write) from [<c03594d4>] (kernfs_fop_write+0xc0/0x180)
 [<c03594d4>] (kernfs_fop_write) from [<c02fc5c8>] (vfs_write+0xa0/0x1a8)
 [<c02fc5c8>] (vfs_write) from [<c02fc9d4>] (SyS_write+0x40/0x8c)
 [<c02fc9d4>] (SyS_write) from [<c020f0a0>] (ret_fast_syscall+0x0/0x48)


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