[PATCH] x86/resctrl: Fix potential lockdep warning

[PATCH] x86/resctrl: Fix potential lockdep warning

[Xiaochen Shen]

rdtgroup_cpus_write() and mkdir_rdt_prepare() call
rdtgroup_kn_lock_live() -> kernfs_to_rdtgroup() to get 'rdtgrp', and
then call rdt_last_cmd_xxx() functions which will check if
rdtgroup_mutex is held/requires its caller to hold rdtgroup_mutex.
But if 'rdtgrp' returned from kernfs_to_rdtgroup() is NULL,
rdtgroup_mutex is not held and calling rdt_last_cmd_xxx() will result
in a lockdep warning.

Remove rdt_last_cmd_xxx() in these two paths. Just returning error
should be sufficient to report to the user that the entry doesn't exist
any more.

Fixes: 94457b36e8a5 ("x86/intel_rdt: Add diagnostics when writing the cpus file")
Fixes: cfd0f34e4cd5 ("x86/intel_rdt: Add diagnostics when making directories")
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index a46dee8e78db..2e3b06d6bbc6 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -461,10 +461,8 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
 	}
 
 	rdtgrp = rdtgroup_kn_lock_live(of->kn);
-	rdt_last_cmd_clear();
 	if (!rdtgrp) {
 		ret = -ENOENT;
-		rdt_last_cmd_puts("Directory was removed\n");
 		goto unlock;
 	}
 
@@ -2648,10 +2646,8 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
 	int ret;
 
 	prdtgrp = rdtgroup_kn_lock_live(prgrp_kn);
-	rdt_last_cmd_clear();
 	if (!prdtgrp) {
 		ret = -ENODEV;
-		rdt_last_cmd_puts("Directory was removed\n");
 		goto out_unlock;
 	}
 
-- 
1.8.3.1

Re: [PATCH] x86/resctrl: Fix potential lockdep warning

[Borislav Petkov]

On Thu, Nov 07, 2019 at 06:36:36AM +0800, Xiaochen Shen wrote:
> rdtgroup_cpus_write() and mkdir_rdt_prepare() call
> rdtgroup_kn_lock_live() -> kernfs_to_rdtgroup() to get 'rdtgrp', and
> then call rdt_last_cmd_xxx() functions which will check if

Write those names like this:

rdt_last_cmd_{clear,puts,...} but not with an "xxx" which confuses
people unfamiliar with the code.

> rdtgroup_mutex is held/requires its caller to hold rdtgroup_mutex.
> But if 'rdtgrp' returned from kernfs_to_rdtgroup() is NULL,
> rdtgroup_mutex is not held and calling rdt_last_cmd_xxx() will result
> in a lockdep warning.

That's more of a self-incurred lockdep warning. You can't be calling
lockdep_assert_held() after a function which doesn't always grab the
mutex. Looks like the design needs changing here...

> Remove rdt_last_cmd_xxx() in these two paths. Just returning error
> should be sufficient to report to the user that the entry doesn't exist
> any more.

... or that.

In any case, you should consider fixing such patterns in the code as it
looks sub-optimal from where I'm standing.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[tip: x86/urgent] x86/resctrl: Fix potential lockdep warning

[tip-bot2 for Xiaochen Shen]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     c8eafe1495303bfd0eedaa8156b1ee9082ee9642
Gitweb:        https://git.kernel.org/tip/c8eafe1495303bfd0eedaa8156b1ee9082ee9642
Author:        Xiaochen Shen <xiaochen.shen@intel.com>
AuthorDate:    Thu, 07 Nov 2019 06:36:36 +08:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 13 Nov 2019 12:34:44 +01:00

x86/resctrl: Fix potential lockdep warning

rdtgroup_cpus_write() and mkdir_rdt_prepare() call
rdtgroup_kn_lock_live() -> kernfs_to_rdtgroup() to get 'rdtgrp', and
then call the rdt_last_cmd_{clear,puts,...}() functions which will check
if rdtgroup_mutex is held/requires its caller to hold rdtgroup_mutex.

But if 'rdtgrp' returned from kernfs_to_rdtgroup() is NULL,
rdtgroup_mutex is not held and calling rdt_last_cmd_{clear,puts,...}()
will result in a self-incurred, potential lockdep warning.

Remove the rdt_last_cmd_{clear,puts,...}() calls in these two paths.
Just returning error should be sufficient to report to the user that the
entry doesn't exist any more.

 [ bp: Massage. ]

Fixes: 94457b36e8a5 ("x86/intel_rdt: Add diagnostics when writing the cpus file")
Fixes: cfd0f34e4cd5 ("x86/intel_rdt: Add diagnostics when making directories")
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: pei.p.jia@intel.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/1573079796-11713-1-git-send-email-xiaochen.shen@intel.com
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index a46dee8..2e3b06d 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -461,10 +461,8 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
 	}
 
 	rdtgrp = rdtgroup_kn_lock_live(of->kn);
-	rdt_last_cmd_clear();
 	if (!rdtgrp) {
 		ret = -ENOENT;
-		rdt_last_cmd_puts("Directory was removed\n");
 		goto unlock;
 	}
 
@@ -2648,10 +2646,8 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
 	int ret;
 
 	prdtgrp = rdtgroup_kn_lock_live(prgrp_kn);
-	rdt_last_cmd_clear();
 	if (!prdtgrp) {
 		ret = -ENODEV;
-		rdt_last_cmd_puts("Directory was removed\n");
 		goto out_unlock;
 	}
 

Re: [PATCH] x86/resctrl: Fix potential lockdep warning

[Xiaochen Shen]

Hi Boris,

Thank you for your kind code review. Please find my comments inline.

On 11/13/2019 19:44, Borislav Petkov wrote:
> On Thu, Nov 07, 2019 at 06:36:36AM +0800, Xiaochen Shen wrote:
>> rdtgroup_cpus_write() and mkdir_rdt_prepare() call
>> rdtgroup_kn_lock_live() -> kernfs_to_rdtgroup() to get 'rdtgrp', and
>> then call rdt_last_cmd_xxx() functions which will check if
> 
> Write those names like this:
> 
> rdt_last_cmd_{clear,puts,...} but not with an "xxx" which confuses
> people unfamiliar with the code.

OK. I got it. rdt_last_cmd_{clear,puts,printf}().

> 
>> rdtgroup_mutex is held/requires its caller to hold rdtgroup_mutex.
>> But if 'rdtgrp' returned from kernfs_to_rdtgroup() is NULL,
>> rdtgroup_mutex is not held and calling rdt_last_cmd_xxx() will result
>> in a lockdep warning.
> 
> That's more of a self-incurred lockdep warning. You can't be calling
> lockdep_assert_held() after a function which doesn't always grab the
> mutex. Looks like the design needs changing here...

Actually this fix covers all the cases of an audit of the calling paths
of rdt_last_cmd_{clear,puts,printf}(), to make sure we only have the
lockdep_assert_held() in places where we are sure that it must be held.
Please find more background details as below.

> 
>> Remove rdt_last_cmd_xxx() in these two paths. Just returning error
>> should be sufficient to report to the user that the entry doesn't exist
>> any more.
> 
> ... or that.
> 
> In any case, you should consider fixing such patterns in the code as it
> looks sub-optimal from where I'm standing.

I would like to provide more of the background details in the commit
comment in v2 patch:

-------------------
x86/resctrl: Fix potential lockdep warning

rdt_last_cmd_{clear,puts,printf}() call lockdep_assert_held() to assert
that rdtgroup_mutex is held.

During internal review of some other changes we found that there are
code paths that call rdt_last_cmd_{clear,puts}() when the rdtgroup_mutex
is not held.

An audit of calling sequences identified two different cases in
rdtgroup_kn_lock_live() which both returning NULL:
1.'rdtgrp' returned from kernfs_to_rdtgroup() is NULL, rdtgroup_mutex
is not held.
2.'rdtgrp' is being deleted, rdtgroup_mutex is held.

Checking all call sites of rdt_last_cmd_{clear,puts,printf}() found two
code paths where rdtgroup_mutex is not held: rdtgroup_cpus_write() and
mkdir_rdt_prepare().

Fix by removing rdt_last_cmd_{clear,puts}() in these two paths. Just
returning error should be sufficient to report to the user that the
entry doesn't exist any more.

Fixes: 94457b36e8a5 ("x86/intel_rdt: Add diagnostics when writing the 
cpus file")
Fixes: cfd0f34e4cd5 ("x86/intel_rdt: Add diagnostics when making 
directories")
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
-------------------
Updated commit comment to provide additional context on how these were
found.

> 
> Thx.
> 

-- 
Best regards,
Xiaochen

Re: [PATCH] x86/resctrl: Fix potential lockdep warning

[Borislav Petkov]

On Sun, Nov 17, 2019 at 12:13:20AM +0800, Xiaochen Shen wrote:
> Actually this fix covers all the cases of an audit of the calling paths
> of rdt_last_cmd_{clear,puts,printf}(), to make sure we only have the
> lockdep_assert_held() in places where we are sure that it must be held.

That's kinda what I suggested, isn't it?

All I meant was, not to have a

	rdtgroup_kn_lock_live()

call in the code as this function does *not* unconditionally grab the
rdtgroup_mutex. And then call a function which unconditionally checks
whether the mutex is held.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH] x86/resctrl: Fix potential lockdep warning

[Xiaochen Shen]

On 11/18/2019 23:02, Borislav Petkov wrote:
> On Sun, Nov 17, 2019 at 12:13:20AM +0800, Xiaochen Shen wrote:
>> Actually this fix covers all the cases of an audit of the calling paths
>> of rdt_last_cmd_{clear,puts,printf}(), to make sure we only have the
>> lockdep_assert_held() in places where we are sure that it must be held.
> 
> That's kinda what I suggested, isn't it?
> 
> All I meant was, not to have a
> 
> 	rdtgroup_kn_lock_live()
> 
> call in the code as this function does *not* unconditionally grab the
> rdtgroup_mutex. And then call a function which unconditionally checks
> whether the mutex is held.
> 

Hi Boris,

Thank you for your good suggestion. I will try to follow up if we could 
improve the code in call sites of rdtgroup_kn_lock_live() in separate patch.

In my opinion, the potential lockdep issues in all call sites of 
rdt_last_cmd_{clear,puts,...}() have been fixed in this patch.

Thank you.

-- 
Best regards,
Xiaochen