On Thu, 11/09 21:43, Max Reitz wrote: > Draining a BDS may lead to graph modifications, which in turn may result > in it and other BDS being stripped of their current references. If > bdrv_drain_all_begin() and bdrv_drain_all_end() do not keep strong > references themselves, the BDS they are trying to drain (or undrain) may > disappear right under their feet -- or, more specifically, under the > feet of BDRV_POLL_WHILE() in bdrv_drain_recurse(). > > This fixes an occasional hang of iotest 194. > > Signed-off-by: Max Reitz <mre...@redhat.com> > --- > block/io.c | 47 ++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 44 insertions(+), 3 deletions(-) > > diff --git a/block/io.c b/block/io.c > index 3d5ef2cabe..a0a2833e8e 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -340,7 +340,10 @@ void bdrv_drain_all_begin(void) > bool waited = true; > BlockDriverState *bs; > BdrvNextIterator it; > - GSList *aio_ctxs = NULL, *ctx; > + GSList *aio_ctxs = NULL, *ctx, *bs_list = NULL, *bs_list_entry; > + > + /* Must be called from the main loop */ > + assert(qemu_get_current_aio_context() == qemu_get_aio_context()); > > block_job_pause_all(); > > @@ -355,6 +358,12 @@ void bdrv_drain_all_begin(void) > if (!g_slist_find(aio_ctxs, aio_context)) { > aio_ctxs = g_slist_prepend(aio_ctxs, aio_context); > } > + > + /* Keep a strong reference to all root BDS and copy them into > + * an own list because draining them may lead to graph > + * modifications. */ > + bdrv_ref(bs); > + bs_list = g_slist_prepend(bs_list, bs); > } > > /* Note that completion of an asynchronous I/O operation can trigger any > @@ -370,7 +379,11 @@ void bdrv_drain_all_begin(void) > AioContext *aio_context = ctx->data; > > aio_context_acquire(aio_context); > - for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) { > + for (bs_list_entry = bs_list; bs_list_entry; > + bs_list_entry = bs_list_entry->next) > + { > + bs = bs_list_entry->data; > + > if (aio_context == bdrv_get_aio_context(bs)) { > waited |= bdrv_drain_recurse(bs, true); > } > @@ -379,24 +392,52 @@ void bdrv_drain_all_begin(void) > } > } > > + for (bs_list_entry = bs_list; bs_list_entry; > + bs_list_entry = bs_list_entry->next) > + { > + bdrv_unref(bs_list_entry->data); > + } > + > g_slist_free(aio_ctxs); > + g_slist_free(bs_list); > } > > void bdrv_drain_all_end(void) > { > BlockDriverState *bs; > BdrvNextIterator it; > + GSList *bs_list = NULL, *bs_list_entry; > + > + /* Must be called from the main loop */ > + assert(qemu_get_current_aio_context() == qemu_get_aio_context()); > > + /* Keep a strong reference to all root BDS and copy them into an > + * own list because draining them may lead to graph modifications. > + */ > for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) { > - AioContext *aio_context = bdrv_get_aio_context(bs); > + bdrv_ref(bs); > + bs_list = g_slist_prepend(bs_list, bs); > + } > + > + for (bs_list_entry = bs_list; bs_list_entry; > + bs_list_entry = bs_list_entry->next) > + { > + AioContext *aio_context; > + > + bs = bs_list_entry->data; > + aio_context = bdrv_get_aio_context(bs); > > aio_context_acquire(aio_context); > aio_enable_external(aio_context); > bdrv_parent_drained_end(bs); > bdrv_drain_recurse(bs, false); > aio_context_release(aio_context); > + > + bdrv_unref(bs); > } > > + g_slist_free(bs_list); > + > block_job_resume_all(); > } > > -- > 2.13.6 > >
It is better to put the references into BdrvNextIterator and introduce bdrv_next_iterator_destroy() to free them? You'll need to touch all callers because it is not C++, but it secures all of rest, which seems vulnerable in the same pattern, for example the aio_poll() in iothread_stop_all(). Fam