[PATCH v4 17/19] jobs: protect job.aio_context with BQL and job_mutex

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

In order to make it thread safe, implement a "fake rwlock",
where we allow reads under BQL *or* job_mutex held, but
writes only under BQL *and* job_mutex.

The only write we have is in child_job_set_aio_ctx, which always
happens under drain (so the job is paused).
For this reason, introduce job_set_aio_context and make sure that
the context is set under BQL, job_mutex and drain.
Also make sure all other places where the aiocontext is read
are protected.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/replication.c |  2 +-
 blockjob.c          |  3 ++-
 include/qemu/job.h  | 19 ++++++++++++++++++-
 job.c               | 12 ++++++++++++
 4 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 50ea778937..68018948b9 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -148,8 +148,8 @@ static void replication_close(BlockDriverState *bs)
     }
     if (s->stage == BLOCK_REPLICATION_FAILOVER) {
         commit_job = &s->commit_job->job;
-        assert(commit_job->aio_context == qemu_get_current_aio_context());
         WITH_JOB_LOCK_GUARD() {
+            assert(commit_job->aio_context == qemu_get_current_aio_context());
             job_cancel_sync_locked(commit_job, false);
         }
     }
diff --git a/blockjob.c b/blockjob.c
index 04d868f020..afb0e9ad5e 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -154,12 +154,13 @@ static void child_job_set_aio_ctx(BdrvChild *c, AioContext *ctx,
         bdrv_set_aio_context_ignore(sibling->bs, ctx, ignore);
     }
 
-    job->job.aio_context = ctx;
+    job_set_aio_context(&job->job, ctx);
 }
 
 static AioContext *child_job_get_parent_aio_context(BdrvChild *c)
 {
     BlockJob *job = c->opaque;
+    assert(qemu_in_main_thread());
 
     return job->job.aio_context;
 }
diff --git a/include/qemu/job.h b/include/qemu/job.h
index fc57472e84..f335bac46c 100644
--- a/include/qemu/job.h
+++ b/include/qemu/job.h
@@ -75,7 +75,12 @@ typedef struct Job {
     ProgressMeter progress;
 
 
-    /** AioContext to run the job coroutine in */
+    /**
+     * AioContext to run the job coroutine in.
+     * This field can be read when holding either the BQL (so we are in
+     * the main loop) or the job_mutex.
+     * Instead, it can be only written when we hold *both* BQL and job_mutex.
+     */
     AioContext *aio_context;
 
     /** Reference count of the block job */
@@ -731,4 +736,16 @@ void job_dismiss_locked(Job **job, Error **errp);
 int job_finish_sync_locked(Job *job, void (*finish)(Job *, Error **errp),
                            Error **errp);
 
+/**
+ * Sets the @job->aio_context.
+ * Called with job_mutex *not* held.
+ *
+ * This function must run in the main thread to protect against
+ * concurrent read in job_finish_sync_locked(),
+ * takes the job_mutex lock to protect against the read in
+ * job_do_yield_locked(), and must be called when the coroutine
+ * is quiescent.
+ */
+void job_set_aio_context(Job *job, AioContext *ctx);
+
 #endif
diff --git a/job.c b/job.c
index bce76b21d8..c4e0641bd4 100644
--- a/job.c
+++ b/job.c
@@ -357,6 +357,17 @@ Job *job_get_locked(const char *id)
     return NULL;
 }
 
+void job_set_aio_context(Job *job, AioContext *ctx)
+{
+    /* protect against read in job_finish_sync_locked */
+    assert(qemu_in_main_thread());
+    /* protect against read in job_do_yield_locked */
+    JOB_LOCK_GUARD();
+    /* ensure the coroutine is quiescent while the AioContext is changed */
+    assert(job->pause_count > 0);
+    job->aio_context = ctx;
+}
+
 /* Called with job_mutex *not* held. */
 static void job_sleep_timer_cb(void *opaque)
 {
@@ -1274,6 +1285,7 @@ int job_finish_sync_locked(Job *job, void (*finish)(Job *, Error **errp),
 {
     Error *local_err = NULL;
     int ret;
+    assert(qemu_in_main_thread());
 
     job_ref_locked(job);
 
-- 
2.31.1