On Dec 8, 2018, at 10:51 PM, Guenter Roeck [off-list ref] wrote:

Hi,

quoted

On Thu, Nov 29, 2018 at 06:12:33PM -0700, Jens Axboe wrote:
sbitmap maintains a set of words that we use to set and clear bits, with
each bit representing a tag for blk-mq. Even though we spread the bits
out and maintain a hint cache, one particular bit allocated will end up
being cleared in the exact same spot.

This introduces batched clearing of bits. Instead of clearing a given
bit, the same bit is set in a cleared/free mask instead. If we fail
allocating a bit from a given word, then we check the free mask, and
batch move those cleared bits at that time. This trades 64 atomic bitops
for 2 cmpxchg().

In a threaded poll test case, half the overhead of getting and clearing
tags is removed with this change. On another poll test case with a
single thread, performance is unchanged.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

This patch results in irq lock inversion warnings when trying to boot
from usb drives. This was observed with qemu boots of aarch64, x86, and
x86_64 images.

This one is a false positive, was already reported last week. Just need to add some lockdep annotation for it.