On Mon, 17 Jun 2024 06:15:25 -0400 Jeff Layton wrote: > We've had number of these reports recently. I think I understand what's > happening but I'm not sure how to fix it. The problem manifests as a > stuck nfsd_mutex: > > nfsd_nl_rpc_status_get_start takes the nfsd_mutex, and it's released in > nfsd_nl_rpc_status_get_done. These are the ->start and ->done > operations for the rpc_status_get dumpit routine. > > I think syzbot is triggering one of the two "goto errout_skb" > conditions in netlink_dump (not sure which). In those cases we end up > returning from that function without calling ->done, which would lead > to the hung mutex like we see here. > > Is this a bug in the netlink code, or is the rpc_status_get dumpit > routine not using ->start and ->done correctly? Dumps are spread over multiple recvmsg() calls, even if we error out the next recvmsg() will dump again, until ->done() is called. And we'll call ->done() if socket is closed without reaching the end. But the multi-syscall nature puts us at the mercy of the user meaning that holding locks ->start() to ->done() is a bit of a no-no. Many of the dumps dump contents of an Xarray, so its easy to remember an index and continue dumping from where we left off.
