locking/pvqspinlock: Fix div by 0 in qstats

PATCH In review Davidlohr Bueso 3 participants 14 messages Apr 18, 2016 – May 5, 2016 1 message not in the archive Locking primitives

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Davidlohr Bueso

author

2016-04-18 06:34 UTC #0 lore raw

PATCH 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 <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

kernel/locking/qspinlock_stat.h +5 -3

index eb2a2c9bc3fc..d734b7502001 100644
@@ -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

Signature · 1 line

2.8.1

Davidlohr Bueso

author

2016-04-18 06:34 UTC #1 lore raw

PATCH 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 <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 2 --
 1 file changed, 2 deletions(-)

kernel/locking/qspinlock_stat.h +0 -2

index d734b7502001..72722334237a 100644
@@ -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;
 }

Signature · 1 line

2.8.1

Waiman Long

2016-04-18 20:14 UTC #2 lore raw

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

Quoted 13 lines · Davidlohr Bueso

... remove the redundant second iteration, this is most likely a copy/past buglet.
Signed-off-by Davidlohr Bueso<[email protected]>
---
  kernel/locking/qspinlock_stat.h | 2 --
  1 file changed, 2 deletions(-)
kernel/locking/qspinlock_stat.h +0 -0
index d734b7502001..72722334237a 100644

kernel/locking/qspinlock_stat.h static ssize_t qstat_write(struct file *file, const char __user *user_buf, · line 196

Quoted 8 lines

kernel/locking/qspinlock_stat.h +0 -2

@@ -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

tip-bot for Davidlohr Bueso

merge bot

2016-05-05 09:44 UTC #3 lore raw

[tip:locking/core] locking/pvqspinlock: Avoid double resetting of stats

In reply to Davidlohr Bueso

locking/pvqspinlock: Avoid double resetting of stats

Commit-ID:  dc209a3fd73ec96d4491bcc128c3b50b0a8e8017
Gitweb:     http://git.kernel.org/tip/dc209a3fd73ec96d4491bcc128c3b50b0a8e8017
Author:     Davidlohr Bueso <[email protected]>
AuthorDate: Sun, 17 Apr 2016 23:31:42 -0700
Committer:  Ingo Molnar <[email protected]>
CommitDate: Thu, 5 May 2016 09:58:49 +0200

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 <[email protected]>

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Linus Torvalds <[email protected]>

Cc Paul E. McKenney <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc [email protected]

Link http://lkml.kernel.org/r/[email protected] open here

Signed-off-by Ingo Molnar <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 2 --
 1 file changed, 2 deletions(-)

kernel/locking/qspinlock_stat.h +0 -2

index d734b75..7272233 100644
@@ -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;
 }

Davidlohr Bueso

author

2016-04-18 06:34 UTC #4 lore raw

PATCH 3/3 locking/pvqspinlock: Robustify init_qspinlock_stat()

In reply to Davidlohr Bueso

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 <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

kernel/locking/qspinlock_stat.h +10 -4

index 72722334237a..ddcd653c942c 100644
@@ -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);

Signature · 1 line

2.8.1

Davidlohr Bueso

author

2016-04-18 16:16 UTC #5 lore raw

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

Quoted 14 lines · Davidlohr Bueso

kernel/locking/qspinlock_stat.h +4 -2

index 72722334237a..ddcd653c942c 100644
@@ -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;
+

kernel/locking/qspinlock_stat.h static int __init init_qspinlock_stat(void) · line 239

Quoted 10 lines

kernel/locking/qspinlock_stat.h +6 -2

+	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

A message that is not in the archive (sent off-list or not yet received): CS1PR84MB0312315E60265F5E8972CACBF16C0@CS1PR84MB0312.NAMPRD84.PROD.OUTLOOK.COM

Davidlohr Bueso

author

2016-04-20 04:10 UTC #7 lore raw

PATCH v2 1/1 locking/pvqspinlock: Robustify init_qspinlock_stat()

Signed-off-by Davidlohr Bueso <[email protected]>

---
kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------

1 file changed, 14 insertions(+), 8 deletions(-)

kernel/locking/qspinlock_stat.h +0 -0

index 72722334237a..fa0863423a04 100644
@@ -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

Davidlohr Bueso

author

2016-04-20 04:13 UTC #8 lore raw

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

Review

Quoted 1 line

(quoted diff) +1 -0

+	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*.

Davidlohr Bueso

author

2016-04-20 04:17 UTC #9 lore raw

PATCH v3 3/3 locking/pvqspinlock: Robustify init_qspinlock_stat()

In reply to a message not in the archive

locking/pvqspinlock: Robustify init_qspinlock_stat()

Signed-off-by Davidlohr Bueso <[email protected]>

---
kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------

1 file changed, 14 insertions(+), 8 deletions(-)

kernel/locking/qspinlock_stat.h +0 -0

index 72722334237a..22e025309845 100644
@@ -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[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);

Signature · 1 line

2.8.1

Waiman Long

Reviewed-by on v3 3/3

2016-04-20 20:07 UTC #10 lore raw

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

Quoted 45 lines · Davidlohr Bueso

locking/pvqspinlock: Robustify init_qspinlock_stat()

Signed-off-by Davidlohr Bueso <[email protected]>

---
kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------

1 file changed, 14 insertions(+), 8 deletions(-)

kernel/locking/qspinlock_stat.h +9 -6

index 72722334237a..22e025309845 100644
@@ -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;

kernel/locking/qspinlock_stat.h static int __init init_qspinlock_stat(void) · line 242

Quoted 10 lines

kernel/locking/qspinlock_stat.h +5 -2

-    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 <[email protected]>

tip-bot for Davidlohr Bueso

merge bot

2016-05-05 09:44 UTC #11 lore raw

[tip:locking/core] locking/pvqspinlock: Robustify init_qspinlock_stat()

In reply to Davidlohr Bueso

locking/pvqspinlock: Robustify init_qspinlock_stat()

Commit-ID:  b96bbdde19cc56f288372d25fd5ea7af04fc1271
Gitweb:     http://git.kernel.org/tip/b96bbdde19cc56f288372d25fd5ea7af04fc1271
Author:     Davidlohr Bueso <[email protected]>
AuthorDate: Tue, 19 Apr 2016 21:17:25 -0700
Committer:  Ingo Molnar <[email protected]>
CommitDate: Thu, 5 May 2016 09:58:51 +0200

locking/pvqspinlock: Robustify init_qspinlock_stat()

Signed-off-by Davidlohr Bueso <[email protected]>

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Reviewed-by Waiman Long <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Linus Torvalds <[email protected]>

Cc Paul E. McKenney <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Thomas Gleixner <[email protected]>

Link http://lkml.kernel.org/r/[email protected] open here

Signed-off-by Ingo Molnar <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

kernel/locking/qspinlock_stat.h +14 -8

index 7272233..22e0253 100644
@@ -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);

Waiman Long

Reviewed-by on v1 3/3

2016-04-18 19:40 UTC #12 lore raw

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

In reply to Davidlohr Bueso

Quoted 27 lines · Davidlohr Bueso

Signed-off-by Davidlohr Bueso<[email protected]>

---
  kernel/locking/qspinlock_stat.h | 14 ++++++++++----
  1 file changed, 10 insertions(+), 4 deletions(-)

kernel/locking/qspinlock_stat.h +7 -2

index 72722334237a..ddcd653c942c 100644
@@ -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;

kernel/locking/qspinlock_stat.h static int __init init_qspinlock_stat(void) · line 242

Quoted 10 lines

kernel/locking/qspinlock_stat.h +3 -2

 
-	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 <[email protected]>

Cheers,
Longman

Waiman Long

Reviewed-by on v1 1/3

2016-04-18 19:34 UTC #13 lore raw

In reply to Davidlohr Bueso

Quoted 57 lines · Davidlohr Bueso

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 ]---

Signed-off-by Davidlohr Bueso<[email protected]>

---
  kernel/locking/qspinlock_stat.h | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

kernel/locking/qspinlock_stat.h +1 -1

index eb2a2c9bc3fc..d734b7502001 100644
@@ -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;

kernel/locking/qspinlock_stat.h static ssize_t qstat_read(struct file *file, char __user *user_buf, · line 147

Quoted 10 lines

kernel/locking/qspinlock_stat.h +4 -2

 
-		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 <[email protected]>

Cheers,
Longman

tip-bot for Davidlohr Bueso

merge bot

2016-04-19 09:35 UTC #14 lore raw

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

In reply to Davidlohr Bueso

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

Commit-ID:  6687659568e2ec5b3ac24b39c5d26ce8b9d90434
Gitweb:     http://git.kernel.org/tip/6687659568e2ec5b3ac24b39c5d26ce8b9d90434
Author:     Davidlohr Bueso <[email protected]>
AuthorDate: Sun, 17 Apr 2016 23:31:41 -0700
Committer:  Ingo Molnar <[email protected]>
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

Signed-off-by Davidlohr Bueso <[email protected]>

Reviewed-by Waiman Long <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Linus Torvalds <[email protected]>

Cc Paul E. McKenney <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc [email protected]

Link http://lkml.kernel.org/r/[email protected] open here

Signed-off-by Ingo Molnar <[email protected]>

---
 kernel/locking/qspinlock_stat.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

kernel/locking/qspinlock_stat.h +5 -3

index eb2a2c9..d734b75 100644
@@ -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