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authorMatteo Croce <mcroce@microsoft.com>2021-07-13 02:05:25 +0300
committerJens Axboe <axboe@kernel.dk>2021-08-02 22:37:28 +0300
commitcf179948554a2e0d2b622317bf6bf33138ac36e5 (patch)
treeb550929f672a4bb4054839099c4bc2a4e2efa0af /block/disk-events.c
parent2164877c7f373e14e55fca20b7c4a9c436fe4462 (diff)
downloadlinux-cf179948554a2e0d2b622317bf6bf33138ac36e5.tar.xz
block: add disk sequence number
Associating uevents with block devices in userspace is difficult and racy: the uevent netlink socket is lossy, and on slow and overloaded systems has a very high latency. Block devices do not have exclusive owners in userspace, any process can set one up (e.g. loop devices). Moreover, device names can be reused (e.g. loop0 can be reused again and again). A userspace process setting up a block device and watching for its events cannot thus reliably tell whether an event relates to the device it just set up or another earlier instance with the same name. Being able to set a UUID on a loop device would solve the race conditions. But it does not allow to derive orderings from uevents: if you see a uevent with a UUID that does not match the device you are waiting for, you cannot tell whether it's because the right uevent has not arrived yet, or it was already sent and you missed it. So you cannot tell whether you should wait for it or not. Associating a unique, monotonically increasing sequential number to the lifetime of each block device, which can be retrieved with an ioctl immediately upon setting it up, allows to solve the race conditions with uevents, and also allows userspace processes to know whether they should wait for the uevent they need or if it was dropped and thus they should move on. Additionally, increment the disk sequence number when the media change, i.e. on DISK_EVENT_MEDIA_CHANGE event. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Matteo Croce <mcroce@microsoft.com> Tested-by: Luca Boccassi <bluca@debian.org> Link: https://lore.kernel.org/r/20210712230530.29323-2-mcroce@linux.microsoft.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/disk-events.c')
-rw-r--r--block/disk-events.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/block/disk-events.c b/block/disk-events.c
index a75931ff5da4..04c52f3992ed 100644
--- a/block/disk-events.c
+++ b/block/disk-events.c
@@ -190,6 +190,9 @@ static void disk_check_events(struct disk_events *ev,
spin_unlock_irq(&ev->lock);
+ if (events & DISK_EVENT_MEDIA_CHANGE)
+ inc_diskseq(disk);
+
/*
* Tell userland about new events. Only the events listed in
* @disk->events are reported, and only if DISK_EVENT_FLAG_UEVENT