linux/block
Tejun Heo 6e736be7f2 block: make ioc get/put interface more conventional and fix race on alloction
Ignoring copy_io() during fork, io_context can be allocated from two
places - current_io_context() and set_task_ioprio().  The former is
always called from local task while the latter can be called from
different task.  The synchornization between them are peculiar and
dubious.

* current_io_context() doesn't grab task_lock() and assumes that if it
  saw %NULL ->io_context, it would stay that way until allocation and
  assignment is complete.  It has smp_wmb() between alloc/init and
  assignment.

* set_task_ioprio() grabs task_lock() for assignment and does
  smp_read_barrier_depends() between "ioc = task->io_context" and "if
  (ioc)".  Unfortunately, this doesn't achieve anything - the latter
  is not a dependent load of the former.  ie, if ioc itself were being
  dereferenced "ioc->xxx", it would mean something (not sure what tho)
  but as the code currently stands, the dependent read barrier is
  noop.

As only one of the the two test-assignment sequences is task_lock()
protected, the task_lock() can't do much about race between the two.
Nothing prevents current_io_context() and set_task_ioprio() allocating
its own ioc for the same task and overwriting the other's.

Also, set_task_ioprio() can race with exiting task and create a new
ioc after exit_io_context() is finished.

ioc get/put doesn't have any reason to be complex.  The only hot path
is accessing the existing ioc of %current, which is simple to achieve
given that ->io_context is never destroyed as long as the task is
alive.  All other paths can happily go through task_lock() like all
other task sub structures without impacting anything.

This patch updates ioc get/put so that it becomes more conventional.

* alloc_io_context() is replaced with get_task_io_context().  This is
  the only interface which can acquire access to ioc of another task.
  On return, the caller has an explicit reference to the object which
  should be put using put_io_context() afterwards.

* The functionality of current_io_context() remains the same but when
  creating a new ioc, it shares the code path with
  get_task_io_context() and always goes through task_lock().

* get_io_context() now means incrementing ref on an ioc which the
  caller already has access to (be that an explicit refcnt or implicit
  %current one).

* PF_EXITING inhibits creation of new io_context and once
  exit_io_context() is finished, it's guaranteed that both ioc
  acquisition functions return %NULL.

* All users are updated.  Most are trivial but
  smp_read_barrier_depends() removal from cfq_get_io_context() needs a
  bit of explanation.  I suppose the original intention was to ensure
  ioc->ioprio is visible when set_task_ioprio() allocates new
  io_context and installs it; however, this wouldn't have worked
  because set_task_ioprio() doesn't have wmb between init and install.
  There are other problems with this which will be fixed in another
  patch.

* While at it, use NUMA_NO_NODE instead of -1 for wildcard node
  specification.

-v2: Vivek spotted contamination from debug patch.  Removed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2011-12-14 00:33:38 +01:00
..
blk-cgroup.c block: make ioc get/put interface more conventional and fix race on alloction 2011-12-14 00:33:38 +01:00
blk-cgroup.h block: fix a typo in the blk-cgroup.h file 2011-10-24 16:08:38 +02:00
blk-core.c block, cfq: move cfqd->cic_index to q->id 2011-12-14 00:33:37 +01:00
blk-exec.c block: add missing blk_queue_dead() checks 2011-12-14 00:33:37 +01:00
blk-flush.c blk-flush: move the queue kick into 2011-10-24 16:24:31 +02:00
blk-integrity.c block: add export.h to files using EXPORT_SYMBOL/THIS_MODULE macros 2011-10-31 19:31:12 -04:00
blk-ioc.c block: make ioc get/put interface more conventional and fix race on alloction 2011-12-14 00:33:38 +01:00
blk-iopoll.c tree-wide: fix assorted typos all over the place 2009-12-04 15:39:55 +01:00
blk-lib.c block: fix patch import error in max_discard_sectors check 2011-07-23 20:34:59 +02:00
blk-map.c block: Always check length of all iov entries in blk_rq_map_user_iov() 2011-11-13 19:58:09 +01:00
blk-merge.c block: attempt to merge with existing requests on plug flush 2011-03-21 10:14:27 +01:00
blk-settings.c block: Fix discard topology stacking and reporting 2011-05-18 10:37:35 +02:00
blk-softirq.c block: Don't check QUEUE_FLAG_SAME_COMP in __blk_complete_request 2011-09-14 09:31:01 +02:00
blk-sysfs.c block, cfq: move cfqd->cic_index to q->id 2011-12-14 00:33:37 +01:00
blk-tag.c block: warn if tag is greater than real_max_depth. 2011-10-25 10:20:05 +02:00
blk-throttle.c block: add blk_queue_dead() 2011-12-14 00:33:37 +01:00
blk-timeout.c fault-injection: add ability to export fault_attr in arbitrary directory 2011-08-03 14:25:20 -10:00
blk.h block: make ioc get/put interface more conventional and fix race on alloction 2011-12-14 00:33:38 +01:00
bsg-lib.c block: Change module.h -> export.h in bsg-lib.c 2011-10-31 19:31:13 -04:00
bsg.c bsg: fix address space warning from sparse 2011-06-20 13:27:45 +02:00
cfq-iosched.c block: make ioc get/put interface more conventional and fix race on alloction 2011-12-14 00:33:38 +01:00
cfq.h blk-cgroup: Add unaccounted time to timeslice_used. 2011-03-12 16:54:00 +01:00
compat_ioctl.c compat_ioctl: fix warning caused by qemu 2011-07-01 22:32:26 +02:00
deadline-iosched.c iosched: prevent aliased requests from starving other I/O 2011-06-02 21:19:05 +02:00
elevator.c block: fix request_queue lifetime handling by making blk_queue_cleanup() properly shutdown 2011-10-19 14:42:16 +02:00
genhd.c block: Revert "[SCSI] genhd: add a new attribute "alias" in gendisk" 2011-11-10 09:03:55 +01:00
ioctl.c Merge branch 'modsplit-Oct31_2011' of git://git.kernel.org/pub/scm/linux/kernel/git/paulg/linux 2011-11-06 19:44:47 -08:00
Kconfig block: add bsg helper library 2011-07-31 22:05:09 +02:00
Kconfig.iosched blk-cgroup: config options re-arrangement 2010-04-26 19:27:56 +02:00
Makefile block: add bsg helper library 2011-07-31 22:05:09 +02:00
noop-iosched.c block: remove per-queue plugging 2011-03-10 08:52:07 +01:00
scsi_ioctl.c block: drop unnecessary blk_get/put_queue() in scsi_cmd_ioctl() and blk_get_tg() 2011-10-19 14:31:25 +02:00