On Tue, Aug 03, 2010 at 02:56:15PM -0400, Trond Myklebust wrote: > On Tue, 2010-08-03 at 14:48 -0400, J. Bruce Fields wrote: > > On Tue, Aug 03, 2010 at 02:40:15PM -0400, Trond Myklebust wrote: > > > On Tue, 2010-08-03 at 13:24 -0400, Jeff Layton wrote: > > > > We got a report recently about a panic in RHEL5 (2.6.18 based kernel). > > > > The problem appears to be that a task_cleanup workqueue job ran and got > > > > passed a pointer to an xprt that had been freed. The bug is here in > > > > case anyone is interested in the details: > > > > > > > > https://bugzilla.redhat.com/show_bug.cgi?id=611938 > > > > > > > > The situation seems to be pretty difficult to reproduce, but I don't > > > > see anything that's intended to ensure that this doesn't occur in RHEL5 > > > > or mainline. The task_cleanup workqueue job doesn't hold a reference to > > > > the xprt, and the job isn't canceled when the xprt is torn down. > > > > > > > > Bruce had a look and suggested that we may need something like the > > > > patch below (pasted in, so it probably won't apply correctly). I've > > > > tested a backported version of it on RHEL5 and it seems to work fine. > > > > > > > > Is it reasonable to cancel task_cleanup when destroying the xprt? Or, > > > > am I missing something that should prevent this situation in mainline > > > > (and perhaps isn't in RHEL5's kernel). > > > > > > > > Any help is appreciated... > > > > > > > > -----------------------------[snip]--------------------------------- > > > > diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c > > > > index dcd0132..2a1f664 100644 > > > > --- a/net/sunrpc/xprt.c > > > > +++ b/net/sunrpc/xprt.c > > > > @@ -1129,6 +1129,7 @@ static void xprt_destroy(struct kref *kref) > > > > rpc_destroy_wait_queue(&xprt->sending); > > > > rpc_destroy_wait_queue(&xprt->resend); > > > > rpc_destroy_wait_queue(&xprt->backlog); > > > > + cancel_work_sync(&xprt->task_cleanup); > > > > /* > > > > * Tear down transport state and free the rpc_xprt > > > > */ > > > > -----------------------------[snip]--------------------------------- > > > > > > > > Thanks, > > > > > > How about doing a wait_on_bit_lock(&xprt->state, XPRT_LOCKED,...) > > > instead? > > > > I'll believe you, but I don't get it: what guarantees that XPRT_LOCKED > > isn't cleared while xprt_autoclose autoclose is still running? > > Look more closely: the callers that enqueue task_cleanup on the > workqueue all ensure that XPRT_LOCKED is held, and it isn't released > until the call to xprt_release_write() at the very end of > xprt_autoclose(). I see that (assuming snd_task is always NULL--but, OK, I think I see why that's true), but we still dereference the xprt after returning from xprt_release_write(). Only to drop the transport_lock, but I don't see why there isn't a race. > The rpciod_workqueue deadlocking is an issue, though, so unfortunately, > we won't be able to make use of the above. Yeah, OK, it was just bugging me that I didn't feel like I understood how XPRT_LOCKED is used. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
