dm snapshot: fix invalidation deadlock

Process persistent exception store metadata IOs in a separate thread.

A snapshot may become invalid while inside generic_make_request().
A synchronous write is then needed to update the metadata while still
inside that function.  Since the introduction of
md-dm-reduce-stack-usage-with-stacked-block-devices.patch this has to
be performed by a separate thread to avoid deadlock.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 43 insertions, 5 deletions

diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
index cb05b744deaa..8c25c2ff724a 100644
--- a/drivers/md/dm-exception-store.c
+++ b/drivers/md/dm-exception-store.c
@@ -125,6 +125,8 @@ struct pstore {
         uint32_t callback_count;
         struct commit_callback *callbacks;
         struct dm_io_client *io_client;
+
+        struct workqueue_struct *metadata_wq;
 };
 
 static unsigned sectors_to_pages(unsigned sectors)
@@ -156,10 +158,24 @@ static void free_area(struct pstore *ps)
         ps->area = NULL;
 }
 
+struct mdata_req {
+        struct io_region *where;
+        struct dm_io_request *io_req;
+        struct work_struct work;
+        int result;
+};
+
+static void do_metadata(struct work_struct *work)
+{
+        struct mdata_req *req = container_of(work, struct mdata_req, work);
+
+        req->result = dm_io(req->io_req, 1, req->where, NULL);
+}
+
 /*
  * Read or write a chunk aligned and sized block of data from a device.
  */
-static int chunk_io(struct pstore *ps, uint32_t chunk, int rw)
+static int chunk_io(struct pstore *ps, uint32_t chunk, int rw, int metadata)
 {
         struct io_region where = {
                 .bdev = ps->snap->cow->bdev,
@@ -173,8 +189,23 @@ static int chunk_io(struct pstore *ps, uint32_t chunk, int rw)
                 .client = ps->io_client,
                 .notify.fn = NULL,
         };
+        struct mdata_req req;
+
+        if (!metadata)
+                return dm_io(&io_req, 1, &where, NULL);
 
-        return dm_io(&io_req, 1, &where, NULL);
+        req.where = &where;
+        req.io_req = &io_req;
+
+        /*
+         * Issue the synchronous I/O from a different thread
+         * to avoid generic_make_request recursion.
+         */
+        INIT_WORK(&req.work, do_metadata);
+        queue_work(ps->metadata_wq, &req.work);
+        flush_workqueue(ps->metadata_wq);
+
+        return req.result;
 }
 
 /*
@@ -189,7 +220,7 @@ static int area_io(struct pstore *ps, uint32_t area, int rw)
         /* convert a metadata area index to a chunk index */
         chunk = 1 + ((ps->exceptions_per_area + 1) * area);
 
-        r = chunk_io(ps, chunk, rw);
+        r = chunk_io(ps, chunk, rw, 0);
         if (r)
                 return r;
 
@@ -230,7 +261,7 @@ static int read_header(struct pstore *ps, int *new_snapshot)
         if (r)
                 return r;
 
-        r = chunk_io(ps, 0, READ);
+        r = chunk_io(ps, 0, READ, 1);
         if (r)
                 goto bad;
 
@@ -292,7 +323,7 @@ static int write_header(struct pstore *ps)
         dh->version = cpu_to_le32(ps->version);
         dh->chunk_size = cpu_to_le32(ps->snap->chunk_size);
 
-        return chunk_io(ps, 0, WRITE);
+        return chunk_io(ps, 0, WRITE, 1);
 }
 
 /*
@@ -409,6 +440,7 @@ static void persistent_destroy(struct exception_store *store)
 {
         struct pstore *ps = get_info(store);
 
+        destroy_workqueue(ps->metadata_wq);
         dm_io_client_destroy(ps->io_client);
         vfree(ps->callbacks);
         free_area(ps);
@@ -588,6 +620,12 @@ int dm_create_persistent(struct exception_store *store)
         atomic_set(&ps->pending_count, 0);
         ps->callbacks = NULL;
 
+        ps->metadata_wq = create_singlethread_workqueue("ksnaphd");
+        if (!ps->metadata_wq) {
+                DMERR("couldn't start header metadata update thread");
+                return -ENOMEM;
+        }
+
         store->destroy = persistent_destroy;
         store->read_metadata = persistent_read_metadata;
         store->prepare_exception = persistent_prepare;