Negative "ios_in_flight" in the 2.4 kernel

[M. Edward Borasky]

I've been looking at some "iostat" data from a 2.4.26 machine. The
device utilizations are 100 percent, even when the disk is idle, which
is mathematically impossible. By doing some digging, I discovered this
is a kernel bug, caused by "hd->ios_in_flight" going negative. The
relevant code appears to my untrained eyes to be in
drivers/block/ll_rw_blk.c, specifically


static inline void down_ios(struct hd_struct *hd)
{
        disk_round_stats(hd);
        --hd->ios_in_flight;
}

static inline void up_ios(struct hd_struct *hd)
{
        disk_round_stats(hd);
        ++hd->ios_in_flight;
}

Question: wouldn't a simple refusal to decrement ios_in_flight in
"down_ios" if it's zero fix this, or am I missing something?

Ed Borasky
znmeb@xxxxxxxxxxx

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