Date:	Wed, 30 Jul 2014 02:03:46 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lenny Szubowicz <lszubowi@...hat.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

On Tuesday, July 29, 2014 07:46:02 AM Prarit Bhargava wrote:
> [v2]: Fixed wording of commit, and added additional testing information.
> 
> A while ago we added a test to mimic some of our users' userspace governor
> programs which monitor system behaviour and will switch governors on the
> fly.  The decision process for this includes looking at time of day,
> expected system load, etc.  For some time now we have had reports of
> system panics in the cpufreq code when using the userspace governor
> utility.
> 
> The userspace utility writes
> /sys/devices/system/cpu/cpuX/cpufreq/scaling_governor and sets the
> governor.  In some cases this can happen rapidly, and under heavy load
> there are occasions where the changes are delayed.  This can mean that
> several governor changes may occur within a short period of time.
> 
> This has exposed a bug in the store_scaling_governor path.  When the sysfs
> file is written to,
> 
> store()
> 	->down_write(&policy->rwsem);
> 	->store_scaling_governor()
> 		-> cpufreq_set_policy()
> 			up_write(&policy->rwsem);
> 			__cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
> 			down_write(&policy->rwsem);
> 
> The release of the policy->rwsem results in a situation where another
> write to the scaling_governor file can now start and overwrite pointers
> and cause corruption.
> 
> ex)
> 
>  Oops: 0000 [#1] SMP
>  Modules linked in: sg e1000e igb iTCO_wdt iTCO_vendor_support coretemp kvm_intel ptp kvm pps_core i7300_edac edac_core ses lpc_ich enclosure ioatdma mfd_core shpchp dca i2c_i801 serio_raw acpi_cpufreq pcspkr mperf nfsd auth_rpcgss nfs_acl lockd sunrpc xfs libcrc32c sr_mod cdrom sd_mod ata_generic crc_t10dif crct10dif_common pata_acpi radeon i2c_algo_bit drm_kms_helper ttm drm ata_piix libata i2c_core megaraid_sas dm_mirror dm_region_hash dm_log dm_mod
>  CPU: 7 PID: 565 Comm: kworker/7:2 Not tainted 3.10.0-121.el7.x86_64 #1
>  Hardware name: Intel MP Server/S7000FC4UR, BIOS SFC4UR.86B.01.00.0031.021820102042 02/18/2010
>  Workqueue: events od_dbs_timer
>  task: ffff88046544cfa0 ti: ffff880465438000 task.ti: ffff880465438000
>  RIP: 0010:[<ffffffff81489bed>]  [<ffffffff81489bed>] od_dbs_timer+0x2d/0x160
>  RSP: 0018:ffff880465439de0  EFLAGS: 00010282
>  RAX: ffff88007f03d500 RBX: 0000000000010de0 RCX: 0000000000000006
>  RDX: 0000000000000006 RSI: 7040000000000000 RDI: ffff88046f2f0e08
>  e1000e 0000:06:00.0: irq 79 for MSI/MSI-X
>  IPv6: ADDRCONF(NETDEV_UP): enp6s0f0: link is not ready
>  RBP: ffff880465439e18 R08: ffff88046f2f0e10 R09: dfedcb48370f0e08
>  R10: dfedcb48370f0e08 R11: 0000000000000000 R12: ffff880465a69200
>  R13: 0000000000000000 R14: ffff88046f2f0e08 R15: 00000000000001c0
>  e1000e 0000:06:00.1: irq 80 for MSI/MSI-X
>  FS:  0000000000000000(0000) GS:ffff88046f2e0000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>  CR2: 0000000000000010 CR3: 0000000466be5000 CR4: 00000000000007e0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  Stack:
>   ffff88045ef8d140 ffff880468e1ad80 ffff88046f2f0e08 ffff880465a69200
>   ffff88046f2f3e00 ffff88046f2f7f00 00000000000001c0 ffff880465439e60
>   ffffffff8107e02b 000000006f2f3e18 0000000000000000 ffff88046f2f3e18
>  Call Trace:
>   [<ffffffff8107e02b>] process_one_work+0x17b/0x460
>   [<ffffffff8107edfb>] worker_thread+0x11b/0x400
>   [<ffffffff8107ece0>] ? rescuer_thread+0x400/0x400
>   [<ffffffff81085aef>] kthread+0xcf/0xe0
>   [<ffffffff81085a20>] ? kthread_create_on_node+0x140/0x140
>  e1000e 0000:06:00.1: irq 80 for MSI/MSI-X
>  IPv6: ADDRCONF(NETDEV_UP): enp6s0f1: link is not ready
>   [<ffffffff815fc76c>] ret_from_fork+0x7c/0xb0
>   [<ffffffff81085a20>] ? kthread_create_on_node+0x140/0x140
>  Code: 66 66 90 55 48 89 e5 41 57 41 56 49 89 fe 41 55 41 54 53 48 c7 c3 e0 0d 01 00 48 83 ec 10 48 8b 47 f8 8b 50 14 4c 8b 68 40 89 d1 <4d> 8b 7d 10 89 55 cc 48 03 1c cd 80 78 9e 81 48 8d 83 a8 00 00
>  RIP  [<ffffffff81489bed>] od_dbs_timer+0x2d/0x160
>   RSP <ffff880465439de0>
>  CR2: 0000000000000010
>  BUG: unable to handle kernel  ---[ end trace c9a1ca82e01a4294 ]---
>  Kernel panic - not syncing: Fatal exception
> NULL pointer dereference at 0000000000000014
>  IP: [<ffffffff81489be4>] od_dbs_timer+0x24/0x160
>  PGD 46661d067 PUD 465540067 PMD 0
>  Oops: 0000 [#2] SMP
>  Modules linked in: sg e1000e igb iTCO_wdt iTCO_vendor_support coretemp kvm_intel ptp kvm pps_core i7300_edac edac_core ses lpc_ich enclosure ioatdma mfd_core shpchp dca i2c_i801 serio_raw acpi_cpufreq pcspkr mperf nfsd auth_rpcgss nfs_acl lockd sunrpc xfs libcrc32c sr_mod cdrom sd_mod ata_generic crc_t10dif crct10dif_common pata_acpi radeon i2c_algo_bit drm_kms_helper ttm drm ata_piix libata i2c_core megaraid_sas dm_mirror dm_region_hash dm_log dm_mod
>  CPU: 6 PID: 506 Comm: kworker/6:2 Tainted: G      D     --------------   3.10.0-121.el7.x86_64 #1
>  Hardware name: Intel MP Server/S7000FC4UR, BIOS SFC4UR.86B.01.00.0031.021820102042 02/18/2010
>  Workqueue: events od_dbs_timer
>  task: ffff880465448b60 ti: ffff880463730000 task.ti: ffff880463730000
>  RIP: 0010:[<ffffffff81489be4>]  [<ffffffff81489be4>] od_dbs_timer+0x24/0x160
>  RSP: 0018:ffff880463731de0  EFLAGS: 00010282
>  RAX: 0000000000000000 RBX: 0000000000010de0 RCX: dead000000200200
>  RDX: 0000000000000001 RSI: 7040000000000000 RDI: ffff88046f2d0e08
>  RBP: ffff880463731e18 R08: ffff88046f2d0e10 R09: dfedcb50370d0e08
>  R10: dfedcb50370d0e08 R11: 0000000000000000 R12: ffff880465bd2b80
>  R13: ffff88046f2d3e00 R14: ffff88046f2d0e08 R15: 0000000000000180
>  FS:  0000000000000000(0000) GS:ffff88046f2c0000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>  CR2: 0000000000000014 CR3: 0000000466be5000 CR4: 00000000000007e0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  Stack:
>   ffff88045ef8d140 ffff880468dcf1c0 ffff88046f2d0e08 ffff880465bd2b80
>   ffff88046f2d3e00 ffff88046f2d7f00 0000000000000180 ffff880463731e60
>   ffffffff8107e02b 000000006f2d3e18 0000000000000000 ffff88046f2d3e18
>  Call Trace:
>   [<ffffffff8107e02b>] process_one_work+0x17b/0x460
>   [<ffffffff8107edfb>] worker_thread+0x11b/0x400
>   [<ffffffff8107ece0>] ? rescuer_thread+0x400/0x400
>   [<ffffffff81085aef>] kthread+0xcf/0xe0
>   [<ffffffff81085a20>] ? kthread_create_on_node+0x140/0x140
>   [<ffffffff815fc76c>] ret_from_fork+0x7c/0xb0
>   [<ffffffff81085a20>] ? kthread_create_on_node+0x140/0x140
>  Code: 1f 84 00 00 00 00 00 66 66 66 66 90 55 48 89 e5 41 57 41 56 49 89 fe 41 55 41 54 53 48 c7 c3 e0 0d 01 00 48 83 ec 10 48 8b 47 f8 <8b> 50 14 4c 8b 68 40 89 d1 4d 8b 7d 10 89 55 cc 48 03 1c cd 80
>  RIP  [<ffffffff81489be4>] od_dbs_timer+0x24/0x160
>   RSP <ffff880463731de0>
>  CR2: 0000000000000014
>  drm_kms_helper: panic occurred, switching back to text console
> 
> There are other panics associated with this, including list corruption and
> various flavors of panics in the od_dbs_timer & work code.
> 
> As a test I introduced a separate mutex around the cpufreq_set_policy() call.
> After doing so system behaves as expected, and the panics no longer happen.
> I narrowed the mutex down to the "governor switch" code in
> cpufreq_set_policy() and saw that everything stabilized.
> 
> I then reverted commit 955ef4833574636819cd269cfbae12f79cbde63a, cpufreq: Drop
> rwsem lock around CPUFREQ_GOV_POLICY_EXIT, which added the dropping of the
> policy->rwsem and again, the panics no longer occurred.
> 
> The closer I looked at commit 955ef483, the more questions I have.  The first
> thing is that it appears that the stacktrace includes function calls that
> are not, and never have been, part of the linux.git tree, ie) the call trace
> shows
> 
>             [<c0055a79>] lock_acquire+0x61/0xbc
>             [<c00fabf1>] sysfs_addrm_finish+0xc1/0x128
>             [<c00f9819>] sysfs_hash_and_remove+0x35/0x64
>             [<c00fbe6f>] remove_files.isra.0+0x1b/0x24
>             [<c00fbea5>] sysfs_remove_group+0x2d/0xa8
>             [<c02f9a0b>] cpufreq_governor_interactive+0x13b/0x35c
>             [<c02f61df>] __cpufreq_governor+0x2b/0x8c
>             [<c02f6579>] __cpufreq_set_policy+0xa9/0xf8
>             [<c02f6b75>] store_scaling_governor+0x61/0x100
>             [<c02f6f4d>] store+0x39/0x60
>             [<c00f9b81>] sysfs_write_file+0xed/0x114
>             [<c00b3fd1>] vfs_write+0x65/0xd8
>             [<c00b424b>] sys_write+0x2f/0x50
>             [<c000cdc1>] ret_fast_syscall+0x1/0x52
> 
> and the top part of the stack from cpufreq_governor_interactive() appear to
> be for a driver that is not in the linux.git tree.  Google search does show
> that it exists in various android trees, however, my concern is that the "core"
> scaling governors are now broken because of an out tree driver.
> 
> It seems like the patch fixed this problem incorrectly by breaking the existing
> locking.  The whole point of the policy->rwsem is to single thread updates
> to the policy struct; this is no longer the case after that commit as we
> are breaking the locking right in the middle of a policy->governor update.
> The fix should have been to the cpufreq_governor_interactive out-of-tree
> driver and fix the locking there in its release code.  In any case, the current
> linux.git code no longer can reproduce the original failure; the locking in the
> sysfs release code has changed.
> 
> In case anyone is interested in reproducing, the reproducer I used was
> 
> i=0
> while [ True ]; do
>         i=$((i+1))
>         echo "ondemand" > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor &
>         echo "performance" > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor &
>         echo "ondemand" > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor  &
>         echo "performance" > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor &
>         if [ $((i % 100)) = 0 ]; then
>                 num_ps=$(ps aux | grep doit | wc -l)
>                 echo "$i running, $num_ps outstanding"
>         fi
> done
> 
> and the system usually panics within 500 iterations.
> 
> I have tested this extensively across 15 systems of various configurations.
> In the unpatched kernel I see oopses usually within 500 iterations.  In the
> patched kernel which fixes the broken locking, I can go several hours
> without any issues.
> 
> This patch effectively reverts commit 955ef483.

OK, I'm convinced by this.

I suppose we should push it for -stable from 3.10 through 3.15.x, right?

> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: Lenny Szubowicz <lszubowi@...hat.com>
> Cc: linux-pm@...r.kernel.org
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> ---
>  drivers/cpufreq/cpufreq.c |    8 +-------
>  include/linux/cpufreq.h   |    4 ----
>  2 files changed, 1 insertion(+), 11 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 6f02485..4869f0e 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2198,12 +2198,8 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>  	/* save old, working values */
>  	old_gov = policy->governor;
>  	/* end old governor */
> -	if (old_gov) {
> -		__cpufreq_governor(policy, CPUFREQ_GOV_STOP);
> -		up_write(&policy->rwsem);
> +	if (old_gov)
>  		__cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
> -		down_write(&policy->rwsem);
> -	}
>  
>  	/* start new governor */
>  	policy->governor = new_policy->governor;
> @@ -2211,9 +2207,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>  		if (!__cpufreq_governor(policy, CPUFREQ_GOV_START))
>  			goto out;
>  
> -		up_write(&policy->rwsem);
>  		__cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
> -		down_write(&policy->rwsem);
>  	}
>  
>  	/* new governor failed, so re-start old one */
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 8f8ae95..b450c43 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -100,10 +100,6 @@ struct cpufreq_policy {
>  	 * - Any routine that will write to the policy structure and/or may take away
>  	 *   the policy altogether (eg. CPU hotplug), will hold this lock in write
>  	 *   mode before doing so.
> -	 *
> -	 * Additional rules:
> -	 * - Lock should not be held across
> -	 *     __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);
>  	 */
>  	struct rw_semaphore	rwsem;
>  
> 

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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