• Jens Axboe's avatar
    dm: don't reuse bio for flushes · dbe3ece1
    Jens Axboe authored
    DM currently has a statically allocated bio that it uses to issue empty
    flushes. It doesn't submit this bio, it just uses it for maintaining
    state while setting up clones. Multiple users can access this bio at the
    same time. This wasn't previously an issue, even if it was a bit iffy,
    but with the blkg associations it can become one.
    
    We setup the blkg association, then clone bio's and submit, then remove
    the blkg assocation again. But since we can have multiple tasks doing
    this at the same time, against multiple blkg's, then we can either lose
    references to a blkg, or put it twice. The latter causes complaints on
    the percpu ref being <= 0 when released, and can cause use-after-free as
    well. Ming reports that xfstest generic/475 triggers this:
    
    ------------[ cut here ]------------
    percpu ref (blkg_release) <= 0 (0) after switching to atomic
    WARNING: CPU: 13 PID: 0 at lib/percpu-refcount.c:155 percpu_ref_switch_to_atomic_rcu+0x2c9/0x4a0
    
    Switch to just using an on-stack bio for this, and get rid of the
    embedded bio.
    
    Fixes: 5cdf2e3f ("blkcg: associate blkg when associating a device")
    Reported-by: default avatarMing Lei <ming.lei@redhat.com>
    Tested-by: default avatarMing Lei <ming.lei@redhat.com>
    Reviewed-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    dbe3ece1
dm.c 73.6 KB