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Re: [PATCH] dump-guest-memory: Use BQL to protect dump finalize process
From: |
Marc-André Lureau |
Subject: |
Re: [PATCH] dump-guest-memory: Use BQL to protect dump finalize process |
Date: |
Tue, 16 Nov 2021 11:43:14 +0400 |
Hi
On Tue, Nov 16, 2021 at 7:22 AM Peter Xu <peterx@redhat.com> wrote:
>
> When finalizing the dump-guest-memory with detached mode, we'll first set dump
> status to either FAIL or COMPLETE before doing the cleanup, however right
> after
> the dump status change it's possible that another dump-guest-memory qmp
> command
> is sent so both the main thread and dump thread (which is during cleanup)
> could
> be accessing dump state in paralell. That's racy.
parallel
>
> Fix it by protecting the finalizing phase of dump-guest-memory using BQL as
> well. To do that, we expand the BQL from dump_cleanup() into dump_process(),
> so we will take the BQL longer than before. The critical section must cover
> the status switch from ACTIVE->{FAIL|COMPLETE} so as to make sure there's no
> race any more.
>
> We can also just introduce a specific mutex to serialize the dump process, but
> BQL should be enough for now so far, not to mention vm_start() in dump_cleanup
> will need BQL anyway, so maybe easier to just use the same mutex.
>
> Reported-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
> dump/dump.c | 24 ++++++++++++++++++------
> 1 file changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/dump/dump.c b/dump/dump.c
> index 662d0a62cd..196b7b8ab9 100644
> --- a/dump/dump.c
> +++ b/dump/dump.c
> @@ -96,13 +96,7 @@ static int dump_cleanup(DumpState *s)
> g_free(s->guest_note);
> s->guest_note = NULL;
> if (s->resume) {
> - if (s->detached) {
> - qemu_mutex_lock_iothread();
> - }
> vm_start();
> - if (s->detached) {
> - qemu_mutex_unlock_iothread();
> - }
> }
> migrate_del_blocker(dump_migration_blocker);
>
> @@ -1873,6 +1867,11 @@ static void dump_process(DumpState *s, Error **errp)
> Error *local_err = NULL;
> DumpQueryResult *result = NULL;
>
> + /*
> + * When running with detached mode, these operations are not run with
> BQL.
> + * It's still safe, because it's protected by setting s->state to ACTIVE,
> + * so dump_in_progress() check will block yet another dump-guest-memory.
> + */
> if (s->has_format && s->format == DUMP_GUEST_MEMORY_FORMAT_WIN_DMP) {
> #ifdef TARGET_X86_64
> create_win_dump(s, &local_err);
> @@ -1883,6 +1882,15 @@ static void dump_process(DumpState *s, Error **errp)
> create_vmcore(s, &local_err);
> }
>
> + /*
> + * Serialize the finalizing of dump process using BQL to make sure no
> + * concurrent access to DumpState is allowed. BQL is also required for
> + * dump_cleanup as vm_start() needs it.
> + */
> + if (s->detached) {
> + qemu_mutex_lock_iothread();
> + }
> +
> /* make sure status is written after written_size updates */
> smp_wmb();
> qatomic_set(&s->status,
> @@ -1898,6 +1906,10 @@ static void dump_process(DumpState *s, Error **errp)
>
> error_propagate(errp, local_err);
> dump_cleanup(s);
> +
> + if (s->detached) {
> + qemu_mutex_unlock_iothread();
> + }
> }
>
> static void *dump_thread(void *data)
> --
> 2.32.0
>