From 18c6ac1c6eb7cc541249585836659d0d3ed3a539 Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Mon, 5 Oct 2020 17:58:54 +0200 Subject: [PATCH] block: Add bdrv_lock()/unlock() Inside of coroutine context, we can't directly use aio_context_acquire() for the AioContext of a block node because we already own the lock of the current AioContext and we need to avoid double locking to prevent deadlocks. This provides helper functions to lock the AioContext of a node only if it's not the same as the current AioContext. Signed-off-by: Kevin Wolf Message-Id: <20201005155855.256490-14-kwolf@redhat.com> Reviewed-by: Stefan Hajnoczi Signed-off-by: Markus Armbruster --- block.c | 27 +++++++++++++++++++++++++++ include/block/block.h | 14 ++++++++++++++ 2 files changed, 41 insertions(+) diff --git a/block.c b/block.c index 8d9b9017d3..430edf79bb 100644 --- a/block.c +++ b/block.c @@ -6326,6 +6326,33 @@ void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx) bdrv_dec_in_flight(bs); } +void coroutine_fn bdrv_co_lock(BlockDriverState *bs) +{ + AioContext *ctx = bdrv_get_aio_context(bs); + + /* In the main thread, bs->aio_context won't change concurrently */ + assert(qemu_get_current_aio_context() == qemu_get_aio_context()); + + /* + * We're in coroutine context, so we already hold the lock of the main + * loop AioContext. Don't lock it twice to avoid deadlocks. + */ + assert(qemu_in_coroutine()); + if (ctx != qemu_get_aio_context()) { + aio_context_acquire(ctx); + } +} + +void coroutine_fn bdrv_co_unlock(BlockDriverState *bs) +{ + AioContext *ctx = bdrv_get_aio_context(bs); + + assert(qemu_in_coroutine()); + if (ctx != qemu_get_aio_context()) { + aio_context_release(ctx); + } +} + void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co) { aio_co_enter(bdrv_get_aio_context(bs), co); diff --git a/include/block/block.h b/include/block/block.h index 1027c58a41..d16c401cb4 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -657,6 +657,20 @@ AioContext *coroutine_fn bdrv_co_enter(BlockDriverState *bs); */ void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx); +/** + * Locks the AioContext of @bs if it's not the current AioContext. This avoids + * double locking which could lead to deadlocks: This is a coroutine_fn, so we + * know we already own the lock of the current AioContext. + * + * May only be called in the main thread. + */ +void coroutine_fn bdrv_co_lock(BlockDriverState *bs); + +/** + * Unlocks the AioContext of @bs if it's not the current AioContext. + */ +void coroutine_fn bdrv_co_unlock(BlockDriverState *bs); + /** * Transfer control to @co in the aio context of @bs */