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Subject[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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