On Fri, 11/10 14:17, Kevin Wolf wrote: > Am 10.11.2017 um 03:45 hat Fam Zheng geschrieben: > > 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(); > > > } > > > > 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(). > > You could automatically free the references when bdrv_next() returns > NULL. Then you need an explicit bdrv_next_iterator_destroy() only for > callers that stop iterating halfway through the list.
Yes, good idea. > > Do you actually need to keep references to all BDSes in the whole list > while using the iterator or would it be enough to just keep a reference > to the current one? To fix the bug we now see I think keeping the current is enough, but I think implementing just like this patch is also good with some future-proofing: we cannot know what will be wedged into the nexted aio_poll()'s over time (and yes, we should really reduce the number of them.) Fam