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 <[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]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files 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