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authorJens Axboe <axboe@fb.com>2014-04-05 07:26:04 +0400
committerJens Axboe <axboe@fb.com>2014-04-07 18:17:10 +0400
commit60b0ea120c80cba1cf5fe5ae82a35b1179263de3 (patch)
tree9f56334e557e2665d51beff152f292ffd1ed5f79 /block
parent18a1a7a1d862ae0794a0179473d08a414dd49234 (diff)
downloadlinux-60b0ea120c80cba1cf5fe5ae82a35b1179263de3.tar.xz
percpu_counter: fix bad counter state during suspend
I got a bug report yesterday from Laszlo Ersek <lersek@xxxxxxxxxx>, in which he states that his kvm instance fails to suspend. He Laszlo bisected it down to this commit: commit 1cf7e9c68fe84248174e998922b39e508375e7c1 Author: Jens Axboe <axboe@xxxxxxxxx> Date: Fri Nov 1 10:52:52 2013 -0600 virtio_blk: blk-mq support where virtio-blk is converted to use the blk-mq infrastructure. After digging a bit, it became clear that the issue was with the queue drain. blk-mq tracks queue usage in a percpu counter, which is incremented on request alloc and decremented when the request is freed. The initial hunt was for an inconsistency in blk-mq, but everything seemed fine. In fact, the counter only returned crazy values when suspend was in progress. When a CPU is unplugged, the percpu counters merges that CPU state with the general state. blk-mq takes care to register a hotcpu notifier with the appropriate priority, so we know it runs after the percpu counter notifier. However, the percpu counter notifier only merges the state when the CPU is fully gone. This leaves a state transition where the CPU going away is no longer in the online mask, yet it still holds private values. This means that in this state, percpu_counter_sum() returns invalid results, and the suspend then hangs waiting for abs(dead-cpu-value) requests to complete which of course will never happen. Fix this by clearing the state earlier, so we never have a case where the CPU isn't in online mask but still holds private state. This bug has been there since forever, I guess we don't have a lot of users where percpu counters needs to be reliable during the suspend cycle. Reported-by: <lersek@redhat.com> Tested-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Jens Axboe <axboe@fb.com>
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