| Message ID | 1517501761.2746.21.camel@wdc.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi Bart, On 18/2/2 00:16, Bart Van Assche wrote: > On Thu, 2018-02-01 at 09:53 +0800, Joseph Qi wrote: >> I'm afraid the risk may also exist in blk_cleanup_queue, which will >> set queue_lock to to the default internal lock. >> >> spin_lock_irq(lock); >> if (q->queue_lock != &q->__queue_lock) >> q->queue_lock = &q->__queue_lock; >> spin_unlock_irq(lock); >> >> I'm thinking of getting blkg->q->queue_lock to local first, but this >> will result in still using driver lock even the queue_lock has already >> been set to the default internal lock. > > Hello Joseph, > > I think the race between the queue_lock assignment in blk_cleanup_queue() > and the use of that pointer by cgroup attributes could be solved by > removing the visibility of these attributes from blk_cleanup_queue() instead > of __blk_release_queue(). However, last time I proposed to move code from > __blk_release_queue() into blk_cleanup_queue() I received the feedback that > from some kernel developers that they didn't like this. > > Is the block driver that triggered the race on the q->queue_lock assignment > using legacy (single queue) or multiqueue (blk-mq) mode? If that driver is > using legacy mode, are you aware that there are plans to remove legacy mode > from the upstream kernel? And if your driver is using multiqueue mode, how > about the following change instead of the two patches in this patch series: > We triggered this race when using single queue. I'm not sure if it exists in multi-queue. Do you mean upstream won't fix bugs any more in single queue? Thanks, Joseph > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -1093,7 +1093,7 @@ blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id) > return NULL; > > q->request_fn = rfn; > - if (lock) > + if (!q->mq_ops && lock) > q->queue_lock = lock; > if (blk_init_allocated_queue(q) < 0) { > blk_cleanup_queue(q); > > Thanks, > > Bart. >
On 2/1/18 6:02 PM, Joseph Qi wrote: > Hi Bart, > > On 18/2/2 00:16, Bart Van Assche wrote: >> On Thu, 2018-02-01 at 09:53 +0800, Joseph Qi wrote: >>> I'm afraid the risk may also exist in blk_cleanup_queue, which will >>> set queue_lock to to the default internal lock. >>> >>> spin_lock_irq(lock); >>> if (q->queue_lock != &q->__queue_lock) >>> q->queue_lock = &q->__queue_lock; >>> spin_unlock_irq(lock); >>> >>> I'm thinking of getting blkg->q->queue_lock to local first, but this >>> will result in still using driver lock even the queue_lock has already >>> been set to the default internal lock. >> >> Hello Joseph, >> >> I think the race between the queue_lock assignment in blk_cleanup_queue() >> and the use of that pointer by cgroup attributes could be solved by >> removing the visibility of these attributes from blk_cleanup_queue() instead >> of __blk_release_queue(). However, last time I proposed to move code from >> __blk_release_queue() into blk_cleanup_queue() I received the feedback that >> from some kernel developers that they didn't like this. >> >> Is the block driver that triggered the race on the q->queue_lock assignment >> using legacy (single queue) or multiqueue (blk-mq) mode? If that driver is >> using legacy mode, are you aware that there are plans to remove legacy mode >> from the upstream kernel? And if your driver is using multiqueue mode, how >> about the following change instead of the two patches in this patch series: >> > We triggered this race when using single queue. I'm not sure if it > exists in multi-queue. > Do you mean upstream won't fix bugs any more in single queue? No, we'll still fix bugs in the legacy path, we just won't introduce any new features of accept any new drivers that use that path. Ultimately that path will go away once there are no more users, but until then it is maintained.
On Fri, 2018-02-02 at 09:02 +0800, Joseph Qi wrote: > We triggered this race when using single queue. I'm not sure if it > exists in multi-queue. Regarding the races between modifying the queue_lock pointer and the code that uses that pointer, I think the following construct in blk_cleanup_queue() is sufficient to avoid races between the queue_lock pointer assignment and the code that executes concurrently with blk_cleanup_queue(): spin_lock_irq(lock); if (q->queue_lock != &q->__queue_lock) q->queue_lock = &q->__queue_lock; spin_unlock_irq(lock); In other words, I think that this patch series should be sufficient to address all races between .queue_lock assignments and the code that uses that pointer. Thanks, Bart.
Hi Bart, On 18/2/3 00:21, Bart Van Assche wrote: > On Fri, 2018-02-02 at 09:02 +0800, Joseph Qi wrote: >> We triggered this race when using single queue. I'm not sure if it >> exists in multi-queue. > > Regarding the races between modifying the queue_lock pointer and the code that > uses that pointer, I think the following construct in blk_cleanup_queue() is > sufficient to avoid races between the queue_lock pointer assignment and the code > that executes concurrently with blk_cleanup_queue(): > > spin_lock_irq(lock); > if (q->queue_lock != &q->__queue_lock) > q->queue_lock = &q->__queue_lock; > spin_unlock_irq(lock); > IMO, the race also exists. blk_cleanup_queue blkcg_print_blkgs spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5) q->queue_lock = &q->__queue_lock (3) spin_unlock_irq(lock) (4) spin_unlock_irq(blkg->q->queue_lock) (6) (1) take driver lock; (2) busy loop for driver lock; (3) override driver lock with internal lock; (4) unlock driver lock; (5) can take driver lock now; (6) but unlock internal lock. If we get blkg->q->queue_lock to local first like blk_cleanup_queue, it indeed can fix the different lock use in lock/unlock. But since blk_cleanup_queue has overridden queue lock to internal lock now, I'm afraid we couldn't still use driver lock in blkcg_print_blkgs. Thanks, Joseph > In other words, I think that this patch series should be sufficient to address > all races between .queue_lock assignments and the code that uses that pointer. > > Thanks, > > Bart. >
On Sat, 2018-02-03 at 10:51 +0800, Joseph Qi wrote: > Hi Bart, > > On 18/2/3 00:21, Bart Van Assche wrote: > > On Fri, 2018-02-02 at 09:02 +0800, Joseph Qi wrote: > > > We triggered this race when using single queue. I'm not sure if it > > > exists in multi-queue. > > > > Regarding the races between modifying the queue_lock pointer and the code that > > uses that pointer, I think the following construct in blk_cleanup_queue() is > > sufficient to avoid races between the queue_lock pointer assignment and the code > > that executes concurrently with blk_cleanup_queue(): > > > > spin_lock_irq(lock); > > if (q->queue_lock != &q->__queue_lock) > > q->queue_lock = &q->__queue_lock; > > spin_unlock_irq(lock); > > > > IMO, the race also exists. > > blk_cleanup_queue blkcg_print_blkgs > spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5) > q->queue_lock = &q->__queue_lock (3) > spin_unlock_irq(lock) (4) > spin_unlock_irq(blkg->q->queue_lock) (6) > > (1) take driver lock; > (2) busy loop for driver lock; > (3) override driver lock with internal lock; > (4) unlock driver lock; > (5) can take driver lock now; > (6) but unlock internal lock. > > If we get blkg->q->queue_lock to local first like blk_cleanup_queue, > it indeed can fix the different lock use in lock/unlock. But since > blk_cleanup_queue has overridden queue lock to internal lock now, I'm > afraid we couldn't still use driver lock in blkcg_print_blkgs. (+ Jan Kara) Hello Joseph, That's a good catch. Since modifying all code that accesses the queue_lock pointer and that can race with blk_cleanup_queue() would be too cumbersome I see only one solution, namely making the request queue cgroup and sysfs attributes invisible before the queue_lock pointer is restored. Leaving the debugfs attributes visible while blk_cleanup_queue() is in progress should be fine if the request queue initialization code is modified such that it only modifies the queue_lock pointer for legacy queues. Jan, I think some time ago you objected when I proposed to move code from __blk_release_queue() into blk_cleanup_queue(). Would you be fine with a slightly different approach, namely making block cgroup and sysfs attributes invisible earlier, namely from inside blk_cleanup_queue() instead of from inside __blk_release_queue()? Thanks, Bart.
On Mon 05-02-18 17:58:12, Bart Van Assche wrote: > On Sat, 2018-02-03 at 10:51 +0800, Joseph Qi wrote: > > Hi Bart, > > > > On 18/2/3 00:21, Bart Van Assche wrote: > > > On Fri, 2018-02-02 at 09:02 +0800, Joseph Qi wrote: > > > > We triggered this race when using single queue. I'm not sure if it > > > > exists in multi-queue. > > > > > > Regarding the races between modifying the queue_lock pointer and the code that > > > uses that pointer, I think the following construct in blk_cleanup_queue() is > > > sufficient to avoid races between the queue_lock pointer assignment and the code > > > that executes concurrently with blk_cleanup_queue(): > > > > > > spin_lock_irq(lock); > > > if (q->queue_lock != &q->__queue_lock) > > > q->queue_lock = &q->__queue_lock; > > > spin_unlock_irq(lock); > > > > > > > IMO, the race also exists. > > > > blk_cleanup_queue blkcg_print_blkgs > > spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5) > > q->queue_lock = &q->__queue_lock (3) > > spin_unlock_irq(lock) (4) > > spin_unlock_irq(blkg->q->queue_lock) (6) > > > > (1) take driver lock; > > (2) busy loop for driver lock; > > (3) override driver lock with internal lock; > > (4) unlock driver lock; > > (5) can take driver lock now; > > (6) but unlock internal lock. > > > > If we get blkg->q->queue_lock to local first like blk_cleanup_queue, > > it indeed can fix the different lock use in lock/unlock. But since > > blk_cleanup_queue has overridden queue lock to internal lock now, I'm > > afraid we couldn't still use driver lock in blkcg_print_blkgs. > > (+ Jan Kara) > > Hello Joseph, > > That's a good catch. Since modifying all code that accesses the queue_lock > pointer and that can race with blk_cleanup_queue() would be too cumbersome I > see only one solution, namely making the request queue cgroup and sysfs > attributes invisible before the queue_lock pointer is restored. Leaving the > debugfs attributes visible while blk_cleanup_queue() is in progress should > be fine if the request queue initialization code is modified such that it > only modifies the queue_lock pointer for legacy queues. Jan, I think some > time ago you objected when I proposed to move code from __blk_release_queue() > into blk_cleanup_queue(). Would you be fine with a slightly different > approach, namely making block cgroup and sysfs attributes invisible earlier, > namely from inside blk_cleanup_queue() instead of from inside > __blk_release_queue()? Making attributes invisible earlier should be fine. But this whole switching of queue_lock in blk_cleanup_queue() looks error-prone to me. Generally anyone having access to request_queue can have old value of q->queue_lock in its CPU caches and can happily use that value after blk_cleanup_queue() finishes and the driver specific structure storing the lock is freed. blkcg_print_blkgs() is one such example but I wouldn't bet a penny that there are no other paths with a similar problem. Logically, the lifetime of storage for q->queue_lock should be at least as long as that of the request_queue itself - i.e., released only after __blk_release_queue(). Otherwise all users of q->queue_lock need a proper synchronization against lock switch in blk_cleanup_queue(). Either of these looks like a lot of work. I guess since this involves only a legacy path, your approach to move removal sysfs attributes earlier might be a reasonable band aid. Honza
--- a/block/blk-core.c +++ b/block/blk-core.c @@ -1093,7 +1093,7 @@ blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id) return NULL; q->request_fn = rfn; - if (lock) + if (!q->mq_ops && lock) q->queue_lock = lock; if (blk_init_allocated_queue(q) < 0) { blk_cleanup_queue(q);