aio_context_acquire() and aio_context_release() make it possible for multiple
threads to safely operate on a shared AioContext. This is a prerequisite for
using the block layer outside the QEMU global mutex.
Imagine that a dataplane thread is performing I/O on behalf of the guest when
the user issues a monitor command that needs to access the BlockDriverState.
The monitor thread needs to acquire the AioContext before calling bdrv_*()
functions on it. This prevents the dataplane thread from interfering with the
monitor thread.
There was a discussion in the RFC email thread about how to implement fairness:
each thread should get a turn to acquire the AioContext so that starvation is
avoided.
Paolo suggested doing what the QEMU main loop does today: drop the lock before
invoking ppoll(2). It turns out that this is tricky since we want both threads
to be able to call aio_poll() simultaneously. AioContext->pollfds[] currently
prevents two simultaneous aio_poll() calls since it is a shared resource. It's
also tricky to avoid deadlocks if two threads execute aio_poll() simulatenously
except by waking all threads each time *any* thread makes any progress.
I found the simplest solution is to implement RFifoLock, a recursive lock with
FIFO ordering. This lock supports the semantics we need for the following
pattern:
/* Event loop thread */
while (running) {
aio_context_acquire(ctx);
aio_poll(ctx, true);
aio_context_release(ctx);
}
/* Another thread */
aio_context_acquire(ctx);
bdrv_read(bs, 0x1000, buf, 1);
aio_context_release(ctx);
When the other thread wants to acquire the AioContext but the lock is
contended, it uses aio_notify(ctx) to bump the event loop thread out of
aio_poll(). Fairness ensures everyone gets an opportunity to use the
AioContext.
Stefan Hajnoczi (2):
rfifolock: add recursive FIFO lock
aio: add aio_context_acquire() and aio_context_release()
async.c | 18 ++++++++++
include/block/aio.h | 18 ++++++++++
include/qemu/rfifolock.h | 54 +++++++++++++++++++++++++++++
tests/Makefile | 2 ++
tests/test-aio.c | 58 +++++++++++++++++++++++++++++++
tests/test-rfifolock.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++
util/Makefile.objs | 1 +
util/rfifolock.c | 79 ++++++++++++++++++++++++++++++++++++++++++
8 files changed, 320 insertions(+)
create mode 100644 include/qemu/rfifolock.h
create mode 100644 tests/test-rfifolock.c
create mode 100644 util/rfifolock.c
--
1.8.3.1
