On Thu, 2006-12-14 at 22:35 -0800, Chen, Kenneth W wrote:
> Chen, Kenneth wrote on Thursday, December 14, 2006 5:59 PM
> > > It seems utterly insane to have aio_complete() flush a workqueue. That
> > > function has to be called from a number of different environments,
> > > including non-sleep tolerant environments.
> > >
> > > For instance it means that directIO on NFS will now cause the rpciod
> > > workqueues to call flush_workqueue(aio_wq), thus slowing down all RPC
> > > activity.
> >
> > The bug appears to be somewhere else, somehow the ref count on ioctx is
> > all messed up.
> >
> > In aio_complete, __put_ioctx() should not be invoked because ref count
> > on ioctx is supposedly more than 2, aio_complete decrement it once and
> > should return without invoking the free function.
This makes absolutely no sense whatsoever. If the refcount is 'always
supposedo to be more than 2' then why would you need a refcount at all
in aio_complete?
> > The real freeing ioctx should be coming from exit_aio() or io_destroy(),
> > in which case both wait until no further pending AIO request via
> > wait_for_all_aios().
>
> Ah, I think I see the bug: it must be a race between io_destroy() and
> aio_complete(). A possible scenario:
>
> cpu0 cpu1
> io_destroy aio_complete
> wait_for_all_aios { __aio_put_req
> ... ctx->reqs_active--;
> if (!ctx->reqs_active)
> return;
> }
> ...
> put_ioctx(ioctx)
>
> put_ioctx(ctx);
> bam! Bug trigger!
>
> AIO finished on cpu1 and while in the middle of aio_complete, cpu0 starts
> io_destroy sequence, sees no pending AIO, went ahead decrement the ref
> count on ioctx. At a later point in aio_complete, the put_ioctx decrement
> last ref count and calls the ioctx freeing function and there it triggered
> the bug warning.
>
> A simple fix would be to access ctx->reqs_active inside ctx spin lock in
> wait_for_all_aios(). At the mean time, I would like to
> remove ref counting
> for each iocb because we already performing ref count using reqs_active. This
> would also prevent similar buggy code in the future.
>
>
> Signed-off-by: Ken Chen <[EMAIL PROTECTED]>
>
> --- ./fs/aio.c.orig 2006-11-29 13:57:37.000000000 -0800
> +++ ./fs/aio.c 2006-12-14 20:45:14.000000000 -0800
> @@ -298,17 +298,23 @@ static void wait_for_all_aios(struct kio
> struct task_struct *tsk = current;
> DECLARE_WAITQUEUE(wait, tsk);
>
> + spin_lock_irq(&ctx->ctx_lock);
> if (!ctx->reqs_active)
> - return;
> + goto out;
>
> add_wait_queue(&ctx->wait, &wait);
> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> while (ctx->reqs_active) {
> + spin_unlock_irq(&ctx->ctx_lock);
> schedule();
> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> + spin_lock_irq(&ctx->ctx_lock);
> }
> __set_task_state(tsk, TASK_RUNNING);
> remove_wait_queue(&ctx->wait, &wait);
> +
> +out:
> + spin_unlock_irq(&ctx->ctx_lock);
> }
>
> /* wait_on_sync_kiocb:
> @@ -425,7 +431,6 @@ static struct kiocb fastcall *__aio_get_
> ring = kmap_atomic(ctx->ring_info.ring_pages[0], KM_USER0);
> if (ctx->reqs_active < aio_ring_avail(&ctx->ring_info, ring)) {
> list_add(&req->ki_list, &ctx->active_reqs);
> - get_ioctx(ctx);
> ctx->reqs_active++;
> okay = 1;
> }
> @@ -538,8 +543,6 @@ int fastcall aio_put_req(struct kiocb *r
> spin_lock_irq(&ctx->ctx_lock);
> ret = __aio_put_req(ctx, req);
> spin_unlock_irq(&ctx->ctx_lock);
> - if (ret)
> - put_ioctx(ctx);
> return ret;
> }
>
> @@ -795,8 +798,7 @@ static int __aio_run_iocbs(struct kioctx
> */
> iocb->ki_users++; /* grab extra reference */
> aio_run_iocb(iocb);
> - if (__aio_put_req(ctx, iocb)) /* drop extra ref */
> - put_ioctx(ctx);
> + __aio_put_req(ctx, iocb);
> }
> if (!list_empty(&ctx->run_list))
> return 1;
> @@ -942,7 +944,6 @@ int fastcall aio_complete(struct kiocb *
> struct io_event *event;
> unsigned long flags;
> unsigned long tail;
> - int ret;
>
> /*
> * Special case handling for sync iocbs:
> @@ -1011,18 +1012,12 @@ int fastcall aio_complete(struct kiocb *
> pr_debug("%ld retries: %zd of %zd\n", iocb->ki_retried,
> iocb->ki_nbytes - iocb->ki_left, iocb->ki_nbytes);
> put_rq:
> - /* everything turned out well, dispose of the aiocb. */
> - ret = __aio_put_req(ctx, iocb);
> -
> spin_unlock_irqrestore(&ctx->ctx_lock, flags);
>
> if (waitqueue_active(&ctx->wait))
> wake_up(&ctx->wait);
>
> - if (ret)
> - put_ioctx(ctx);
> -
> - return ret;
> + return aio_put_req(iocb);
> }
New race: You are now calling wake_up(&ctx->wait) before you've called
aio_put_req(). You also still have the exact same race as before since
aio_put_req() also calls put_ioctx() if ret != 0...
Trond
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