Ethernet Drivers: Question on ifp->if_ioctl invocation for SIOCADDMULTI and SIOCDELMULTI

David Somayajulu david.somayajulu at
Wed May 16 18:56:46 UTC 2012

Hi All,
When ifp->if_ioctl() is invoked for the ioctl cmd SIOCADDMULTI,

IN_MULTI_LOCK() is called in one of the functions in_joingroup()  in the caller stack.

>From netinet/in_var.h, line 357 :  #define IN_MULTI_LOCK()         mtx_lock(&in_multi_mtx)

>From netinet/in_mcast.c
1098 in_joingroup(struct ifnet *ifp, const struct in_addr *gina,
1099     /*const*/ struct in_mfilter *imf, struct in_multi **pinm)
1100 {
1101         int error;
 1103         IN_MULTI_LOCK();
1104         error = in_joingroup_locked(ifp, gina, imf, pinm);
1105         IN_MULTI_UNLOCK();

This is also the case for SIOCDELMULTI, where the function holding  "in_multi_mtx" lock is in_leavegroup()

This poses a problem in the driver in that the hardware dependent function performing it,  is not allowed to sleep() in case it needs to poll some state.


1.       If I want to implement any delays - for the above case - in the driver using DELAY(usec) macro, is there a maximum amount of time that the driver is allowed to complete this function? I am concerned that if it takes to too long I might run into a soft_lockup() situation.

2.       Is it o.k to defer the processing in a separate in a separate thread which can sleep() ?

David S.

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