From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Fabian Ebner Date: Wed, 25 May 2022 13:59:39 +0200 Subject: [PATCH] PVE-Backup: avoid segfault issues upon backup-cancel When canceling a backup in PVE via a signal it's easy to run into a situation where the job is already failing when the backup_cancel QMP command comes in. With a bit of unlucky timing on top, it can happen that job_exit() runs between schedulung of job_cancel_bh() and execution of job_cancel_bh(). But job_cancel_sync() does not expect that the job is already finalized (in fact, the job might've been freed already, but even if it isn't, job_cancel_sync() would try to deref job->txn which would be NULL at that point). It is not possible to simply use the job_cancel() (which is advertised as being async but isn't in all cases) in qmp_backup_cancel() for the same reason job_cancel_sync() cannot be used. Namely, because it can invoke job_finish_sync() (which uses AIO_WAIT_WHILE and thus hangs if called from a coroutine). This happens when there's multiple jobs in the transaction and job->deferred_to_main_loop is true (is set before scheduling job_exit()) or if the job was not started yet. Fix the issue by selecting the job to cancel in job_cancel_bh() itself using the first job that's not completed yet. This is not necessarily the first job in the list, because pvebackup_co_complete_stream() might not yet have removed a completed job when job_cancel_bh() runs. An alternative would be to continue using only the first job and checking against JOB_STATUS_CONCLUDED or JOB_STATUS_NULL to decide if it's still necessary and possible to cancel, but the approach with using the first non-completed job seemed more robust. Signed-off-by: Fiona Ebner Signed-off-by: Wolfgang Bumiller [FE: adapt for new job lock mechanism replacing AioContext locks] Signed-off-by: Fiona Ebner --- pve-backup.c | 57 ++++++++++++++++++++++++++++++++++------------------ 1 file changed, 38 insertions(+), 19 deletions(-) diff --git a/pve-backup.c b/pve-backup.c index 67e2b99d74..7a8240363d 100644 --- a/pve-backup.c +++ b/pve-backup.c @@ -356,12 +356,41 @@ static void pvebackup_complete_cb(void *opaque, int ret) /* * job_cancel(_sync) does not like to be called from coroutines, so defer to - * main loop processing via a bottom half. + * main loop processing via a bottom half. Assumes that caller holds + * backup_mutex. */ static void job_cancel_bh(void *opaque) { CoCtxData *data = (CoCtxData*)opaque; - Job *job = (Job*)data->data; - job_cancel_sync(job, true); + + /* + * Be careful to pick a valid job to cancel: + * 1. job_cancel_sync() does not expect the job to be finalized already. + * 2. job_exit() might run between scheduling and running job_cancel_bh() + * and pvebackup_co_complete_stream() might not have removed the job from + * the list yet (in fact, cannot, because it waits for the backup_mutex). + * Requiring !job_is_completed() ensures that no finalized job is picked. + */ + GList *bdi = g_list_first(backup_state.di_list); + while (bdi) { + if (bdi->data) { + BlockJob *bj = ((PVEBackupDevInfo *)bdi->data)->job; + if (bj) { + Job *job = &bj->job; + WITH_JOB_LOCK_GUARD() { + if (!job_is_completed_locked(job)) { + job_cancel_sync_locked(job, true); + /* + * It's enough to cancel one job in the transaction, the + * rest will follow automatically. + */ + break; + } + } + } + } + bdi = g_list_next(bdi); + } + aio_co_enter(data->ctx, data->co); } @@ -382,22 +411,12 @@ void coroutine_fn qmp_backup_cancel(Error **errp) proxmox_backup_abort(backup_state.pbs, "backup canceled"); } - /* it's enough to cancel one job in the transaction, the rest will follow - * automatically */ - GList *bdi = g_list_first(backup_state.di_list); - BlockJob *cancel_job = bdi && bdi->data ? - ((PVEBackupDevInfo *)bdi->data)->job : - NULL; - - if (cancel_job) { - CoCtxData data = { - .ctx = qemu_get_current_aio_context(), - .co = qemu_coroutine_self(), - .data = &cancel_job->job, - }; - aio_bh_schedule_oneshot(data.ctx, job_cancel_bh, &data); - qemu_coroutine_yield(); - } + CoCtxData data = { + .ctx = qemu_get_current_aio_context(), + .co = qemu_coroutine_self(), + }; + aio_bh_schedule_oneshot(data.ctx, job_cancel_bh, &data); + qemu_coroutine_yield(); qemu_co_mutex_unlock(&backup_state.backup_mutex); }