CHROMIUM: md: dm-verity: Fix to avoid a deadlock in dm-bufio

Changed the dm-verity prefetching to use a worker thread to avoid
a deadlock in dm-bufio.

If generic_make_request is called recursively, it queues the I/O
request on the current->bio_list without making the I/O request
and returns. The routine making the recursive call cannot wait
for the I/O to complete.

The deadlock occurred when one thread grabbed the bufio_client
mutex and waited for an I/O to complete but the I/O was queued
on another thread's current->bio_list and it was waiting to get
the mutex held by the first thread.

The fix allows only one I/O request from dm-verity to dm-bufio
per thread. To do this, the prefetch requests were queued on worker
threads.

In addition to avoiding the deadlock, this fix made a slight
improvement in performance.

seconds_kernel_to_login:
  with prefetch:    8.43s
  without prefetch: 9.2s
  worker prefetch:  8.28s

BUG=chromium-os:39148
TEST=./run_remote_tests.sh --board=$B --remote=$IP suite:smoke
Signed-off-by: Paul Taysom <taysom@chromium.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@kernel.org
Change-Id: I5f3cddfed507abfb8198fee71e713495f076b772
Reviewed-on: https://gerrit.chromium.org/gerrit/42422
Tested-by: Paul Taysom <taysom@chromium.org>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Queue: Paul Taysom <taysom@chromium.org>

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index cc06a1e..c0fc827 100644 (file)
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -1029,6 +1029,8 @@ void dm_bufio_prefetch(struct dm_bufio_client *c,
 {
        struct blk_plug plug;
 
+       BUG_ON(dm_bufio_in_request());
+
        blk_start_plug(&plug);
        dm_bufio_lock(c);
 

diff --git a/drivers/md/dm-verity.c b/drivers/md/dm-verity.c
index 5bdce4a..cb48412 100644 (file)
--- a/drivers/md/dm-verity.c
+++ b/drivers/md/dm-verity.c
@@ -100,6 +100,13 @@ struct dm_verity_io {
         */
 };
 
+struct dm_verity_prefetch_work {
+       struct work_struct work;
+       struct dm_verity *v;
+       sector_t block;
+       unsigned n_blocks;
+};
+
 /* Provide a lightweight means of specifying the global default for
  * error behavior: eio, reboot, or none
  * Legacy support for 0 = eio, 1 = reboot/panic, 2 = none, 3 = notify.
@@ -581,15 +588,18 @@ static void verity_end_io(struct bio *bio, int error)
  * The root buffer is not prefetched, it is assumed that it will be cached
  * all the time.
  */
-static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
+static void verity_prefetch_io(struct work_struct *work)
 {
+       struct dm_verity_prefetch_work *pw =
+               container_of(work, struct dm_verity_prefetch_work, work);
+       struct dm_verity *v = pw->v;
        int i;
 
        for (i = v->levels - 2; i >= 0; i--) {
                sector_t hash_block_start;
                sector_t hash_block_end;
-               verity_hash_at_level(v, io->block, i, &hash_block_start, NULL);
-               verity_hash_at_level(v, io->block + io->n_blocks - 1, i, &hash_block_end, NULL);
+               verity_hash_at_level(v, pw->block, i, &hash_block_start, NULL);
+               verity_hash_at_level(v, pw->block + pw->n_blocks - 1, i, &hash_block_end, NULL);
                if (!i) {
                        unsigned cluster = *(volatile unsigned *)&dm_verity_prefetch_cluster;
 
@@ -607,8 +617,26 @@ static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
                }
 no_prefetch_cluster:
                dm_bufio_prefetch(v->bufio, hash_block_start,
-                                 hash_block_end - hash_block_start + 1);
+                                  hash_block_end - hash_block_start + 1);
        }
+       kfree(pw);
+}
+
+static void verity_submit_prefetch(struct dm_verity *v, struct dm_verity_io *io)
+{
+       struct dm_verity_prefetch_work *pw;
+
+       pw = kmalloc(sizeof(struct dm_verity_prefetch_work),
+               GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
+
+       if (!pw)
+               return;
+
+       INIT_WORK(&pw->work, verity_prefetch_io);
+       pw->v = v;
+       pw->block = io->block;
+       pw->n_blocks = io->n_blocks;
+       queue_work(v->verify_wq, &pw->work);
 }
 
 /*
@@ -659,7 +687,7 @@ static int verity_map(struct dm_target *ti, struct bio *bio,
        memcpy(io->io_vec, bio_iovec(bio),
               io->io_vec_size * sizeof(struct bio_vec));
 
-       verity_prefetch_io(v, io);
+       verity_submit_prefetch(v, io);
 
        generic_make_request(bio);