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[PATCH -tip 1/3] locking/pvqspinlock: Fix div by 0 in qstats

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  [PATCH -tip 1/3] locking/pvqspinlock: Fix div by 0 in qstats Davidlohr Bueso <dave@stgolabs.net> - 2016-04-18 08:40 +0200
    [PATCH -tip 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat() Davidlohr Bueso <dave@stgolabs.net> - 2016-04-18 08:40 +0200
      Re: [PATCH -tip 3/3] locking/pvqspinlock: Robustify  init_qspinlock_stat() Davidlohr Bueso <dave@stgolabs.net> - 2016-04-18 18:20 +0200
        [PATCH -tip v3 3/3] locking/pvqspinlock: Robustify  init_qspinlock_stat() Davidlohr Bueso <dave@stgolabs.net> - 2016-04-20 06:20 +0200
          Re: [PATCH -tip v3 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat() Waiman Long <waiman.long@hpe.com> - 2016-04-20 22:10 +0200
        Re: [PATCH -tip v2] locking/pvqspinlock: Robustify  init_qspinlock_stat() Davidlohr Bueso <dave@stgolabs.net> - 2016-04-20 06:20 +0200
        [PATCH -tip v2] locking/pvqspinlock: Robustify init_qspinlock_stat() Davidlohr Bueso <dave@stgolabs.net> - 2016-04-20 06:20 +0200
      Re: [PATCH -tip 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat() Waiman Long <waiman.long@hpe.com> - 2016-04-18 21:50 +0200
    [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting of stats Davidlohr Bueso <dave@stgolabs.net> - 2016-04-18 08:40 +0200
      Re: [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting  of stats Waiman Long <waiman.long@hpe.com> - 2016-04-18 22:20 +0200
    Re: [PATCH -tip 1/3] locking/pvqspinlock: Fix div by 0 in qstats Waiman Long <waiman.long@hpe.com> - 2016-04-18 21:40 +0200
    [tip:locking/urgent] locking/pvqspinlock: Fix division by zero in  qstat_read() tip-bot for Davidlohr Bueso <tipbot@zytor.com> - 2016-04-19 11:40 +0200

#1381285 — [PATCH -tip 1/3] locking/pvqspinlock: Fix div by 0 in qstats

While playing with such statistics I ran into the following
splat on a VM when opening pv_hash_hops:

[   25.267962] divide error: 0000 [#1] SMP
...
[   25.268807] CPU: 17 PID: 1018 Comm: cat Not tainted 4.6.0-rc3-debug+ #2
[   25.268853] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20151208_145348-sheep05 04/01/2014
[   25.268930] task: ffff88029a10c040 ti: ffff8800b1e1c000 task.ti: ffff8800b1e1c000
[   25.268979] RIP: 0010:[<ffffffff810b61fe>]  [<ffffffff810b61fe>] qstat_read+0x12e/0x1e0
[   25.269043] RSP: 0018:ffff8800b1e1fde8  EFLAGS: 00010246
[   25.269081] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   25.269128] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81911640
[   25.269175] RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
[   25.269223] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000df00
[   25.269270] R13: 0000000000000000 R14: 0000000000000000 R15: ffff8800b1e1ff28
[   25.269319] FS:  00007f3bd2f88700(0000) GS:ffff8802b5240000(0000) knlGS:0000000000000000
[   25.269372] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   25.269411] CR2: 00007f3bd2ddc008 CR3: 000000002c5f0000 CR4: 00000000000406e0
[   25.269467] Stack:
[   25.269487]  00007f3bd2ddd000 0000000000020000 ffffffff811cad7c ffffffff8119750c
[   25.269549]  ffff88002d08e000 ffffea0000b07140 ffffea0000b07140 ffff88002d08e000
[   25.269609]  ffffffff8118d3b9 ffffffff811937a9 00000000102a46cb ffff8802992beb00
[   25.269670] Call Trace:
[   25.269698]  [<ffffffff811cad7c>] ? mem_cgroup_commit_charge+0x6c/0xd0
[   25.269745]  [<ffffffff8119750c>] ? page_add_new_anon_rmap+0x8c/0xd0
[   25.269791]  [<ffffffff8118d3b9>] ? handle_mm_fault+0x1439/0x1b40
[   25.269834]  [<ffffffff811937a9>] ? do_mmap+0x449/0x550
[   25.269872]  [<ffffffff811d3de3>] ? __vfs_read+0x23/0xd0
[   25.270506]  [<ffffffff811d4ab2>] ? rw_verify_area+0x52/0xd0
[   25.271093]  [<ffffffff811d4bb1>] ? vfs_read+0x81/0x120
[   25.271677]  [<ffffffff811d5f12>] ? SyS_read+0x42/0xa0
[   25.272294]  [<ffffffff815720f6>] ? entry_SYSCALL_64_fastpath+0x1e/0xa8
[   25.272904] Code: 00 48 8b 74 24 50 65 48 33 34 25 28 00 00 00 0f 85 b7 00 00 00 48 83 c4 58 5b 5d 41 5c 41 5d 41 5e 41 5f c3 89 ee 4c 89 f0 31 d2 <48> f7 f6 48 d1 ed 48 8d 5c 24 10 48 8d 3c 92 48 89 c1 31 d2 48
[   25.274347] RIP  [<ffffffff810b61fe>] qstat_read+0x12e/0x1e0
[   25.274885]  RSP <ffff8800b1e1fde8>
[   25.275457] ---[ end trace 8558569eb04480ab ]---

Fix this by verifying that qstat_pv_kick_unlock is in fact non-zero,
similarly to what the qstat_pv_latency_wake case does, as if nothing
else, this can come from resetting the statistics, thus having 0 kicks
should be quite valid in this context.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/qspinlock_stat.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index eb2a2c9bc3fc..d734b7502001 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -136,10 +136,12 @@ static ssize_t qstat_read(struct file *file, char __user *user_buf,
 	}
 
 	if (counter == qstat_pv_hash_hops) {
-		u64 frac;
+		u64 frac = 0;
 
-		frac = 100ULL * do_div(stat, kicks);
-		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		if (kicks) {
+			frac = 100ULL * do_div(stat, kicks);
+			frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		}
 
 		/*
 		 * Return a X.XX decimal number
-- 
2.8.1

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#1381294 — [PATCH -tip 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat()

Specifically around the debugfs file creation calls,
I have no idea if they could ever possibly fail, but
this is core code (debug aside) so lets at least
check the return value and inform anything fishy.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/qspinlock_stat.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 72722334237a..ddcd653c942c 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -225,12 +225,18 @@ static int __init init_qspinlock_stat(void)
 	 * performance.
 	 */
 	for (i = 0; i < qstat_num; i++)
-		debugfs_create_file(qstat_names[i], 0400, d_qstat,
-				   (void *)(long)i, &fops_qstat);
+		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
+					 (void *)(long)i, &fops_qstat))
+			goto fail;
+
+	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
+				 (void *)(long)qstat_reset_cnts, &fops_qstat))
+		goto fail;
 
-	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
-			   (void *)(long)qstat_reset_cnts, &fops_qstat);
 	return 0;
+fail:
+	debugfs_remove_recursive(d_qstat);
+	return 1;
 }
 fs_initcall(init_qspinlock_stat);
 
-- 
2.8.1

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#1381870 — Re: [PATCH -tip 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat()

On Sun, 17 Apr 2016, Davidlohr Bueso wrote:

>diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
>index 72722334237a..ddcd653c942c 100644
>--- a/kernel/locking/qspinlock_stat.h
>+++ b/kernel/locking/qspinlock_stat.h
>@@ -225,12 +225,18 @@ static int __init init_qspinlock_stat(void)
> 	 * performance.
> 	 */
> 	for (i = 0; i < qstat_num; i++)
>-		debugfs_create_file(qstat_names[i], 0400, d_qstat,
>-				   (void *)(long)i, &fops_qstat);
>+		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
>+					 (void *)(long)i, &fops_qstat))
>+			goto fail;
>+
>+	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
>+				 (void *)(long)qstat_reset_cnts, &fops_qstat))
>+		goto fail;
>
>-	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
>-			   (void *)(long)qstat_reset_cnts, &fops_qstat);
> 	return 0;
>+fail:
>+	debugfs_remove_recursive(d_qstat);
>+	return 1;

Hmm actually if debugfs_create_dir() fails firstly, we still return 0, Waiman, did
you mean 1 there, no?

Thanks,
Davidlohr

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#1383038 — [PATCH -tip v3 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat()

locking/pvqspinlock: Robustify init_qspinlock_stat()

Specifically around the debugfs file creation calls,
I have no idea if they could ever possibly fail, but
this is core code (debug aside) so lets at least
check the return value and inform anything fishy.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 72722334237a..22e025309845 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -212,10 +212,8 @@ static int __init init_qspinlock_stat(void)
 	struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
 	int i;
 
-	if (!d_qstat) {
-		pr_warn("Could not create 'qlockstat' debugfs directory\n");
-		return 0;
-	}
+	if (!d_qstat)
+		goto out;
 
 	/*
 	 * Create the debugfs files
@@ -225,12 +223,20 @@ static int __init init_qspinlock_stat(void)
 	 * performance.
 	 */
 	for (i = 0; i < qstat_num; i++)
-		debugfs_create_file(qstat_names[i], 0400, d_qstat,
-				   (void *)(long)i, &fops_qstat);
+		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
+					 (void *)(long)i, &fops_qstat))
+			goto fail_undo;
+
+	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
+				 (void *)(long)qstat_reset_cnts, &fops_qstat))
+		goto fail_undo;
 
-	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
-			   (void *)(long)qstat_reset_cnts, &fops_qstat);
 	return 0;
+fail_undo:
+	debugfs_remove_recursive(d_qstat);
+out:
+	pr_warn("Could not create 'qlockstat' debugfs entries\n");
+	return -ENOMEM;
 }
 fs_initcall(init_qspinlock_stat);
 
-- 
2.8.1

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#1383720 — Re: [PATCH -tip v3 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat()

On 04/20/2016 12:17 AM, Davidlohr Bueso wrote:
> locking/pvqspinlock: Robustify init_qspinlock_stat()
>
> Specifically around the debugfs file creation calls,
> I have no idea if they could ever possibly fail, but
> this is core code (debug aside) so lets at least
> check the return value and inform anything fishy.
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
> kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------
> 1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_stat.h 
> b/kernel/locking/qspinlock_stat.h
> index 72722334237a..22e025309845 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -212,10 +212,8 @@ static int __init init_qspinlock_stat(void)
>     struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
>     int i;
>
> -    if (!d_qstat) {
> -        pr_warn("Could not create 'qlockstat' debugfs directory\n");
> -        return 0;
> -    }
> +    if (!d_qstat)
> +        goto out;
>
>     /*
>      * Create the debugfs files
> @@ -225,12 +223,20 @@ static int __init init_qspinlock_stat(void)
>      * performance.
>      */
>     for (i = 0; i < qstat_num; i++)
> -        debugfs_create_file(qstat_names[i], 0400, d_qstat,
> -                   (void *)(long)i, &fops_qstat);
> +        if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
> +                     (void *)(long)i, &fops_qstat))
> +            goto fail_undo;
> +
> +    if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, 
> d_qstat,
> +                 (void *)(long)qstat_reset_cnts, &fops_qstat))
> +        goto fail_undo;
>
> -    debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
> -               (void *)(long)qstat_reset_cnts, &fops_qstat);
>     return 0;
> +fail_undo:
> +    debugfs_remove_recursive(d_qstat);
> +out:
> +    pr_warn("Could not create 'qlockstat' debugfs entries\n");
> +    return -ENOMEM;
> }
> fs_initcall(init_qspinlock_stat);
>

Reviewed-by: Waiman Long <Waiman.Long@hpe.com>

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#1383039 — Re: [PATCH -tip v2] locking/pvqspinlock: Robustify init_qspinlock_stat()

>+	pr_info("Could not create 'qlockstat' debugfs entries\n");

Please ignore this, I was not meaning to change pr_warn level, this was
simply a stale version. Sending v3 *sigh*.

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#1383041 — [PATCH -tip v2] locking/pvqspinlock: Robustify init_qspinlock_stat()

Specifically around the debugfs file creation calls,
I have no idea if they could ever possibly fail, but
this is core code (debug aside) so lets at least
check the return value and inform anything fishy.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 72722334237a..fa0863423a04 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -212,10 +212,8 @@ static int __init init_qspinlock_stat(void)
	struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
	int i;

-	if (!d_qstat) {
-		pr_warn("Could not create 'qlockstat' debugfs directory\n");
-		return 0;
-	}
+	if (!d_qstat)
+		goto out;

	/*
	 * Create the debugfs files
@@ -225,12 +223,20 @@ static int __init init_qspinlock_stat(void)
	 * performance.
	 */
	for (i = 0; i < qstat_num; i++)
-		debugfs_create_file(qstat_names[i], 0400, d_qstat,
-				   (void *)(long)i, &fops_qstat);
+		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
+					 (void *)(long)i, &fops_qstat))
+			goto fail_undo;
+
+	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
+				 (void *)(long)qstat_reset_cnts, &fops_qstat))
+		goto fail_undo;

-	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
-			   (void *)(long)qstat_reset_cnts, &fops_qstat);
	return 0;
+fail_undo:
+	debugfs_remove_recursive(d_qstat);
+out:
+	pr_info("Could not create 'qlockstat' debugfs entries\n");
+	return -ENOMEM;
 }
 fs_initcall(init_qspinlock_stat);

--
2.8.1

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#1382006 — Re: [PATCH -tip 3/3] locking/pvqspinlock: Robustify init_qspinlock_stat()

On 04/18/2016 02:31 AM, Davidlohr Bueso wrote:
> Specifically around the debugfs file creation calls,
> I have no idea if they could ever possibly fail, but
> this is core code (debug aside) so lets at least
> check the return value and inform anything fishy.
>
> Signed-off-by: Davidlohr Bueso<dbueso@suse.de>
> ---
>   kernel/locking/qspinlock_stat.h | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index 72722334237a..ddcd653c942c 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -225,12 +225,18 @@ static int __init init_qspinlock_stat(void)
>   	 * performance.
>   	 */
>   	for (i = 0; i<  qstat_num; i++)
> -		debugfs_create_file(qstat_names[i], 0400, d_qstat,
> -				   (void *)(long)i,&fops_qstat);
> +		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
> +					 (void *)(long)i,&fops_qstat))
> +			goto fail;
> +
> +	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
> +				 (void *)(long)qstat_reset_cnts,&fops_qstat))
> +		goto fail;
>
> -	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
> -			   (void *)(long)qstat_reset_cnts,&fops_qstat);
>   	return 0;
> +fail:
> +	debugfs_remove_recursive(d_qstat);
> +	return 1;
>   }
>   fs_initcall(init_qspinlock_stat);
>

Reviewed-by: Waiman Long <Waiman.Long@hpe.com>

Cheers,
Longman

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#1381297 — [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting of stats

... remove the redundant second iteration, this is most
likely a copy/past buglet.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/qspinlock_stat.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index d734b7502001..72722334237a 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -191,8 +191,6 @@ static ssize_t qstat_write(struct file *file, const char __user *user_buf,
 
 		for (i = 0 ; i < qstat_num; i++)
 			WRITE_ONCE(ptr[i], 0);
-		for (i = 0 ; i < qstat_num; i++)
-			WRITE_ONCE(ptr[i], 0);
 	}
 	return count;
 }
-- 
2.8.1

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#1382033 — Re: [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting of stats

On 04/18/2016 02:31 AM, Davidlohr Bueso wrote:
> ... remove the redundant second iteration, this is most
> likely a copy/past buglet.
>
> Signed-off-by: Davidlohr Bueso<dbueso@suse.de>
> ---
>   kernel/locking/qspinlock_stat.h | 2 --
>   1 file changed, 2 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index d734b7502001..72722334237a 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -191,8 +191,6 @@ static ssize_t qstat_write(struct file *file, const char __user *user_buf,
>
>   		for (i = 0 ; i<  qstat_num; i++)
>   			WRITE_ONCE(ptr[i], 0);
> -		for (i = 0 ; i<  qstat_num; i++)
> -			WRITE_ONCE(ptr[i], 0);
>   	}
>   	return count;
>   }

The double write is done on purpose. As the statistics count update 
isn't atomic, there is a very small chance (p) that clearing the count 
may happen in the middle of read-modify-write bus transaction. Doing a 
double write will reduce the chance further to p^2. This isn't failsafe, 
but I think is good enough.

However, I don't mind eliminate the double write either as we can always 
view the statistics count after a reset to make sure that they are 
properly cleared.

Cheers,
Longman

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#1381996

On 04/18/2016 02:31 AM, Davidlohr Bueso wrote:
> While playing with such statistics I ran into the following
> splat on a VM when opening pv_hash_hops:
>
> [   25.267962] divide error: 0000 [#1] SMP
> ...
> [   25.268807] CPU: 17 PID: 1018 Comm: cat Not tainted 4.6.0-rc3-debug+ #2
> [   25.268853] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20151208_145348-sheep05 04/01/2014
> [   25.268930] task: ffff88029a10c040 ti: ffff8800b1e1c000 task.ti: ffff8800b1e1c000
> [   25.268979] RIP: 0010:[<ffffffff810b61fe>]  [<ffffffff810b61fe>] qstat_read+0x12e/0x1e0
> [   25.269043] RSP: 0018:ffff8800b1e1fde8  EFLAGS: 00010246
> [   25.269081] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> [   25.269128] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81911640
> [   25.269175] RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
> [   25.269223] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000df00
> [   25.269270] R13: 0000000000000000 R14: 0000000000000000 R15: ffff8800b1e1ff28
> [   25.269319] FS:  00007f3bd2f88700(0000) GS:ffff8802b5240000(0000) knlGS:0000000000000000
> [   25.269372] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   25.269411] CR2: 00007f3bd2ddc008 CR3: 000000002c5f0000 CR4: 00000000000406e0
> [   25.269467] Stack:
> [   25.269487]  00007f3bd2ddd000 0000000000020000 ffffffff811cad7c ffffffff8119750c
> [   25.269549]  ffff88002d08e000 ffffea0000b07140 ffffea0000b07140 ffff88002d08e000
> [   25.269609]  ffffffff8118d3b9 ffffffff811937a9 00000000102a46cb ffff8802992beb00
> [   25.269670] Call Trace:
> [   25.269698]  [<ffffffff811cad7c>] ? mem_cgroup_commit_charge+0x6c/0xd0
> [   25.269745]  [<ffffffff8119750c>] ? page_add_new_anon_rmap+0x8c/0xd0
> [   25.269791]  [<ffffffff8118d3b9>] ? handle_mm_fault+0x1439/0x1b40
> [   25.269834]  [<ffffffff811937a9>] ? do_mmap+0x449/0x550
> [   25.269872]  [<ffffffff811d3de3>] ? __vfs_read+0x23/0xd0
> [   25.270506]  [<ffffffff811d4ab2>] ? rw_verify_area+0x52/0xd0
> [   25.271093]  [<ffffffff811d4bb1>] ? vfs_read+0x81/0x120
> [   25.271677]  [<ffffffff811d5f12>] ? SyS_read+0x42/0xa0
> [   25.272294]  [<ffffffff815720f6>] ? entry_SYSCALL_64_fastpath+0x1e/0xa8
> [   25.272904] Code: 00 48 8b 74 24 50 65 48 33 34 25 28 00 00 00 0f 85 b7 00 00 00 48 83 c4 58 5b 5d 41 5c 41 5d 41 5e 41 5f c3 89 ee 4c 89 f0 31 d2<48>  f7 f6 48 d1 ed 48 8d 5c 24 10 48 8d 3c 92 48 89 c1 31 d2 48
> [   25.274347] RIP  [<ffffffff810b61fe>] qstat_read+0x12e/0x1e0
> [   25.274885]  RSP<ffff8800b1e1fde8>
> [   25.275457] ---[ end trace 8558569eb04480ab ]---
>
> Fix this by verifying that qstat_pv_kick_unlock is in fact non-zero,
> similarly to what the qstat_pv_latency_wake case does, as if nothing
> else, this can come from resetting the statistics, thus having 0 kicks
> should be quite valid in this context.
>
> Signed-off-by: Davidlohr Bueso<dbueso@suse.de>
> ---
>   kernel/locking/qspinlock_stat.h | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index eb2a2c9bc3fc..d734b7502001 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -136,10 +136,12 @@ static ssize_t qstat_read(struct file *file, char __user *user_buf,
>   	}
>
>   	if (counter == qstat_pv_hash_hops) {
> -		u64 frac;
> +		u64 frac = 0;
>
> -		frac = 100ULL * do_div(stat, kicks);
> -		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
> +		if (kicks) {
> +			frac = 100ULL * do_div(stat, kicks);
> +			frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
> +		}
>
>   		/*
>   		 * Return a X.XX decimal number

Reviewed-by: Waiman Long <Waiman.Long@hpe.com>

Cheers,
Longman

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#1382331 — [tip:locking/urgent] locking/pvqspinlock: Fix division by zero in qstat_read()

Commit-ID:  6687659568e2ec5b3ac24b39c5d26ce8b9d90434
Gitweb:     http://git.kernel.org/tip/6687659568e2ec5b3ac24b39c5d26ce8b9d90434
Author:     Davidlohr Bueso <dave@stgolabs.net>
AuthorDate: Sun, 17 Apr 2016 23:31:41 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 19 Apr 2016 10:49:19 +0200

locking/pvqspinlock: Fix division by zero in qstat_read()

While playing with the qstat statistics (in <debugfs>/qlockstat/) I ran into
the following splat on a VM when opening pv_hash_hops:

  divide error: 0000 [#1] SMP
  ...
  RIP: 0010:[<ffffffff810b61fe>]  [<ffffffff810b61fe>] qstat_read+0x12e/0x1e0
  ...
  Call Trace:
    [<ffffffff811cad7c>] ? mem_cgroup_commit_charge+0x6c/0xd0
    [<ffffffff8119750c>] ? page_add_new_anon_rmap+0x8c/0xd0
    [<ffffffff8118d3b9>] ? handle_mm_fault+0x1439/0x1b40
    [<ffffffff811937a9>] ? do_mmap+0x449/0x550
    [<ffffffff811d3de3>] ? __vfs_read+0x23/0xd0
    [<ffffffff811d4ab2>] ? rw_verify_area+0x52/0xd0
    [<ffffffff811d4bb1>] ? vfs_read+0x81/0x120
    [<ffffffff811d5f12>] ? SyS_read+0x42/0xa0
    [<ffffffff815720f6>] ? entry_SYSCALL_64_fastpath+0x1e/0xa8

Fix this by verifying that qstat_pv_kick_unlock is in fact non-zero,
similarly to what the qstat_pv_latency_wake case does, as if nothing
else, this can come from resetting the statistics, thus having 0 kicks
should be quite valid in this context.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Waiman Long <Waiman.Long@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: waiman.long@hpe.com
Link: http://lkml.kernel.org/r/1460961103-24953-1-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/qspinlock_stat.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index eb2a2c9..d734b75 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -136,10 +136,12 @@ static ssize_t qstat_read(struct file *file, char __user *user_buf,
 	}
 
 	if (counter == qstat_pv_hash_hops) {
-		u64 frac;
+		u64 frac = 0;
 
-		frac = 100ULL * do_div(stat, kicks);
-		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		if (kicks) {
+			frac = 100ULL * do_div(stat, kicks);
+			frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+		}
 
 		/*
 		 * Return a X.XX decimal number

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