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SubjectRe: [dm-devel] [PATCH] dm-io: Prevent the danging point of the sync io callback function
On Fri, Jun 27, 2014 at 12:01:30PM +0800, Minfei Huang wrote:
> The io address in callback function will become the danging point,
> cause by the thread of sync io wakes up by other threads
> and return to relieve the io address,

Yes, well found. I prefer the following fix however.

- Joe



Author: Joe Thornber <ejt@redhat.com>
Date: Fri Jun 27 15:49:29 2014 +0100

[dm-io] Fix a race condition in the wake up code for sync_io

There's a race condition between the atomic_dec_and_test(&io->count)
in dec_count() and the waking of the sync_io() thread. If the thread
is spuriously woken immediately after the decrement it may exit,
making the on the stack io struct invalid, yet the dec_count could
still be using it.

There are smaller fixes than the one here (eg, just take the io object
off the stack). But I feel this code could use a clean up.

- simplify dec_count().

- It always calls a callback fn now.
- It always frees the io object back to the pool.

- sync_io()

- Take the io object off the stack and allocate it from the pool the
same as async_io.
- Use a completion object rather than an explicit io_schedule()
loop. The callback triggers the completion.

Reported by: Minfei Huang

diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index 3842ac7..a0982e81 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -10,6 +10,7 @@
#include <linux/device-mapper.h>

#include <linux/bio.h>
+#include <linux/completion.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/sched.h>
@@ -32,7 +33,6 @@ struct dm_io_client {
struct io {
unsigned long error_bits;
atomic_t count;
- struct task_struct *sleeper;
struct dm_io_client *client;
io_notify_fn callback;
void *context;
@@ -111,28 +111,27 @@ static void retrieve_io_and_region_from_bio(struct bio *bio, struct io **io,
* We need an io object to keep track of the number of bios that
* have been dispatched for a particular io.
*---------------------------------------------------------------*/
-static void dec_count(struct io *io, unsigned int region, int error)
+static void complete_io(struct io *io)
{
- if (error)
- set_bit(region, &io->error_bits);
+ unsigned long error_bits = io->error_bits;
+ io_notify_fn fn = io->callback;
+ void *context = io->context;

- if (atomic_dec_and_test(&io->count)) {
- if (io->vma_invalidate_size)
- invalidate_kernel_vmap_range(io->vma_invalidate_address,
- io->vma_invalidate_size);
+ if (io->vma_invalidate_size)
+ invalidate_kernel_vmap_range(io->vma_invalidate_address,
+ io->vma_invalidate_size);

- if (io->sleeper)
- wake_up_process(io->sleeper);
+ mempool_free(io, io->client->pool);
+ fn(error_bits, context);
+}

- else {
- unsigned long r = io->error_bits;
- io_notify_fn fn = io->callback;
- void *context = io->context;
+static void dec_count(struct io *io, unsigned int region, int error)
+{
+ if (error)
+ set_bit(region, &io->error_bits);

- mempool_free(io, io->client->pool);
- fn(r, context);
- }
- }
+ if (atomic_dec_and_test(&io->count))
+ complete_io(io);
}

static void endio(struct bio *bio, int error)
@@ -375,48 +374,49 @@ static void dispatch_io(int rw, unsigned int num_regions,
dec_count(io, 0, 0);
}

+struct sync_io {
+ unsigned long error_bits;
+ struct completion complete;
+};
+
+static void sync_complete(unsigned long error, void *context)
+{
+ struct sync_io *sio = context;
+ sio->error_bits = error;
+ complete(&sio->complete);
+}
+
static int sync_io(struct dm_io_client *client, unsigned int num_regions,
struct dm_io_region *where, int rw, struct dpages *dp,
unsigned long *error_bits)
{
- /*
- * gcc <= 4.3 can't do the alignment for stack variables, so we must
- * align it on our own.
- * volatile prevents the optimizer from removing or reusing
- * "io_" field from the stack frame (allowed in ANSI C).
- */
- volatile char io_[sizeof(struct io) + __alignof__(struct io) - 1];
- struct io *io = (struct io *)PTR_ALIGN(&io_, __alignof__(struct io));
+ struct io *io;
+ struct sync_io sio;

if (num_regions > 1 && (rw & RW_MASK) != WRITE) {
WARN_ON(1);
return -EIO;
}

+ init_completion(&sio.complete);
+
+ io = mempool_alloc(client->pool, GFP_NOIO);
io->error_bits = 0;
atomic_set(&io->count, 1); /* see dispatch_io() */
- io->sleeper = current;
+ io->callback = sync_complete;
+ io->context = &sio;
io->client = client;

io->vma_invalidate_address = dp->vma_invalidate_address;
io->vma_invalidate_size = dp->vma_invalidate_size;

dispatch_io(rw, num_regions, where, dp, io, 1);
-
- while (1) {
- set_current_state(TASK_UNINTERRUPTIBLE);
-
- if (!atomic_read(&io->count))
- break;
-
- io_schedule();
- }
- set_current_state(TASK_RUNNING);
+ wait_for_completion_io(&sio.complete);

if (error_bits)
- *error_bits = io->error_bits;
+ *error_bits = sio.error_bits;

- return io->error_bits ? -EIO : 0;
+ return sio.error_bits ? -EIO : 0;
}

static int async_io(struct dm_io_client *client, unsigned int num_regions,
@@ -434,7 +434,6 @@ static int async_io(struct dm_io_client *client, unsigned int num_regions,
io = mempool_alloc(client->pool, GFP_NOIO);
io->error_bits = 0;
atomic_set(&io->count, 1); /* see dispatch_io() */
- io->sleeper = NULL;
io->client = client;
io->callback = fn;
io->context = context;


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