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bdbuf: Fix race condition with sync active flag

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Bug report by Oleg Kravtsov:

In rtems_bdbuf_swapout_processing() function there is the following
lines:

if (bdbuf_cache.sync_active && !transfered_buffers)
{

    rtems_id sync_requester;
    rtems_bdbuf_lock_cache ();
    ...

}

Here access to bdbuf_cache.sync_active is not protected with anything.
Imagine the following test case:

1. Task1 releases buffer(s) with bdbuf_release_modified() calls;

2. After a while swapout task starts and flushes all buffers;

3. In the end of that swapout flush we are before that part of code, and
assume there is task switching (just before "if (bdbuf_cache.sync_active
&& !transfered_buffers)");

4. Some other task (with higher priority) does bdbuf_release_modified
and rtems_bdbuf_syncdev().

This task successfully gets both locks sync and pool (in
rtems_bdbuf_syncdev() function), sets sync_active to true and starts
waiting for RTEMS_BDBUF_TRANSFER_SYNC event with only sync lock got.

5. Task switching happens again and we are again before "if
(bdbuf_cache.sync_active && !transfered_buffers)".

As the result we check sync_active and we come inside that "if"
statement.

6. The result is that we send RTEMS_BDBUF_TRANSFER_SYNC event! Though
ALL modified messages of that task are not flushed yet!

close #1485
This commit is contained in:
Sebastian Huber
2014-11-27 14:41:17 +01:00
parent 7d015db0c6
commit f0f2a3dc19
1 changed files with 13 additions and 7 deletions
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20
cpukit/libblock/src/bdbuf.c
View File

@@ -2534,9 +2534,15 @@ rtems_bdbuf_swapout_processing (unsigned long timer_delta,
{
rtems_bdbuf_swapout_worker* worker;
bool transfered_buffers = false;
bool sync_active;
rtems_bdbuf_lock_cache ();
/*
* To set this to true you need the cache and the sync lock.
*/
sync_active = bdbuf_cache.sync_active;
/*
* If a sync is active do not use a worker because the current code does not
* cleaning up after. We need to know the buffers have been written when
@@ -2546,7 +2552,7 @@ rtems_bdbuf_swapout_processing (unsigned long timer_delta,
* lock. The simplest solution is to get the main swap out task perform all
* sync operations.
*/
if (bdbuf_cache.sync_active)
if (sync_active)
worker = NULL;
else
{
@@ -2558,16 +2564,16 @@ rtems_bdbuf_swapout_processing (unsigned long timer_delta,
rtems_chain_initialize_empty (&transfer->bds);
transfer->dev = BDBUF_INVALID_DEV;
transfer->syncing = bdbuf_cache.sync_active;
transfer->syncing = sync_active;
/*
* When the sync is for a device limit the sync to that device. If the sync
* is for a buffer handle process the devices in the order on the sync
* list. This means the dev is BDBUF_INVALID_DEV.
*/
if (bdbuf_cache.sync_active)
if (sync_active)
transfer->dev = bdbuf_cache.sync_device;
/*
* If we have any buffers in the sync queue move them to the modified
* list. The first sync buffer will select the device we use.
@@ -2584,7 +2590,7 @@ rtems_bdbuf_swapout_processing (unsigned long timer_delta,
rtems_bdbuf_swapout_modified_processing (&transfer->dev,
&bdbuf_cache.modified,
&transfer->bds,
bdbuf_cache.sync_active,
sync_active,
update_timers,
timer_delta);
@@ -2615,7 +2621,7 @@ rtems_bdbuf_swapout_processing (unsigned long timer_delta,
transfered_buffers = true;
}
if (bdbuf_cache.sync_active && !transfered_buffers)
if (sync_active && !transfered_buffers)
{
rtems_id sync_requester;
rtems_bdbuf_lock_cache ();