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
Signed-off-by: Paul Taysom <taysom@xxxxxxxxxxxx>
---
drivers/md/dm-verity.c | 29 +++++++++++++++++++++++++++--
1 file changed, 27 insertions(+), 2 deletions(-)
diff --git a/drivers/md/dm-verity.c b/drivers/md/dm-verity.c
index 52cde98..7313498 100644
--- a/drivers/md/dm-verity.c
+++ b/drivers/md/dm-verity.c
@@ -93,6 +93,13 @@ struct dm_verity_io {
*/
};
+struct dm_verity_prefetch_work {
+ struct work_struct work;
+ struct dm_bufio_client *bufio;
+ sector_t block;
+ unsigned n_blocks;
+};
+
static struct shash_desc *io_hash_desc(struct dm_verity *v, struct dm_verity_io *io)
{
return (struct shash_desc *)(io + 1);
@@ -419,6 +426,17 @@ static void verity_end_io(struct bio *bio, int error)
queue_work(io->v->verify_wq, &io->work);
}
+
+static void do_verity_prefetch_work(struct work_struct *work)
+{
+ struct dm_verity_prefetch_work *vw =
+ container_of(work, struct dm_verity_prefetch_work, work);
+
+ dm_bufio_prefetch(vw->bufio, vw->block, vw->n_blocks);
+
+ kfree(vw);
+}
+
/*
* Prefetch buffers for the specified io.
* The root buffer is not prefetched, it is assumed that it will be cached
@@ -427,6 +445,7 @@ static void verity_end_io(struct bio *bio, int error)
static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
{
int i;
+ struct dm_verity_prefetch_work *vw;
for (i = v->levels - 2; i >= 0; i--) {
sector_t hash_block_start;
@@ -449,8 +468,14 @@ static void verity_prefetch_io(struct dm_verity *v, struct dm_verity_io *io)
hash_block_end = v->hash_blocks - 1;
}
no_prefetch_cluster:
- dm_bufio_prefetch(v->bufio, hash_block_start,
- hash_block_end - hash_block_start + 1);
+ vw = kmalloc(sizeof(*vw), GFP_KERNEL);
+ if (!vw) /* Just prefetching, ignore errors */
+ return;
+ vw->bufio = v->bufio;
+ vw->block = hash_block_start;
+ vw->n_blocks = hash_block_end - hash_block_start + 1;
+ INIT_WORK(&vw->work, do_verity_prefetch_work);
+ queue_work(v->verify_wq, &vw->work);
}
}
--
1.8.1.3
--
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