q: Memory modified after free in usb2

[Hans Petter Selasky]

On Wednesday 25 March 2009, Weongyo Jeong wrote:
> Hello Hans :),
>
> I think porting uath(4) to usb almost have done that it works well to
> associate with AP and for WPA but I'm suffered from a strange panic after
> detach as follows:
>
>   Memory modified after free 0xc4da3600(508) val=24000000 @ 0xc4da3600
>   panic: Most recently used by USBdev
>
>   cpuid = 0
>   KDB: enter: panic
>   [thread pid 17 tid 100036 ]
>   Stopped at      kdb_enter+0x3a: movl    $0,kdb_why
>
> The detach step is like as follows:
>
>   usb2_transfer_unsetup(sc->sc_xfer, UATH_N_XFERS);
>   ...
>   uath_free_rx_data_list(sc);
>   uath_free_tx_data_list(sc);
>   uath_free_cmd_list(sc, sc->sc_cmd, UATH_CMD_LIST_COUNT);
>
> that I've checked all memory leaks or calls after freeing memory but it
> looks it's not a driver problem.
>
> To solve this problem I modified codes slightly like below:
>
>   usb2_transfer_unsetup(sc->sc_xfer, UATH_N_XFERS);
>   usb2_pause_mtx(NULL, 5 * hz);
>   ...
>   uath_free_rx_data_list(sc);
>   uath_free_tx_data_list(sc);
>   uath_free_cmd_list(sc, sc->sc_cmd, UATH_CMD_LIST_COUNT);
>
> After adding it I couldn't see `Memory modified after free' messages
> anymore.  My question is that I can't understand why adding
> usb2_pause_mtx() helps this symptom?

Did you drain all the taskqueues before unsetup ?

--HPS