I'm running LwIP 1.4.1 and have some questions about the event_callback() in
sockets.c
In my project, I am experiencing a crash related to synchronization in
event_callback() and an application thread calling select(). My project is a
uniprocessor system running an RTOS that implements a static priority
scheduler. SYS_ARCH_PROTECT() is implemented by disabling interrupts.
sys_sem_signal() is implemented using a counting semaphore. TCPIP thread is
higher priority than application threads.
The crash happens when the application thread is waiting in select() and the
TCPIP thread is calling event_callback() to process an event. What's happen is
in the below loop, calling sys_sem_signal() results in a context switch on my
project's RTOS even though application thread is lower priority. The RTOS's
semaphore construct doesn't support priority inheritance/elevation. The
application thread wakes up and finishes the select call, modifying the
select_cb_list. When the context switches back to TCPIP thread, it finishes
the loop iteration and crashes because the select_cb_list has been modified.
What I've done to mitigate the context switch is move the line
last_select_cb_ctr = select_cb_ctr; to the top of the for loop. To me the loop
already had handling for a context switch per iteration, but it only saved the
counter at the end. So now it can handle a switch in the call to
sys_sem_signal() as well.
My questions to whether this is a bug depend on:
1) Should SYS_ARCH_PROTECT() do more than just disable interrupts? Something
that would act as a critical section in the case where a context switch happens?
2) Is it assumed that calling sys_sem_signal() will not cause a voluntary
context switch?
SYS_ARCH_PROTECT(lev);
...
again:
for (scb = select_cb_list; scb != NULL; scb = scb->next) {
if (scb->sem_signalled == 0) {
/* semaphore not signalled yet */
int do_signal = 0;
/* Test this select call for our socket */
if (sock->rcvevent > 0) {
if (scb->readset && FD_ISSET(s, scb->readset)) {
do_signal = 1;
}
}
if (sock->sendevent != 0) {
if (!do_signal && scb->writeset && FD_ISSET(s, scb->writeset)) {
do_signal = 1;
}
}
if (sock->errevent != 0) {
if (!do_signal && scb->exceptset && FD_ISSET(s, scb->exceptset)) {
do_signal = 1;
}
}
if (do_signal) {
scb->sem_signalled = 1;
/* Don't call SYS_ARCH_UNPROTECT() before signaling the semaphore, as
this might
lead to the select thread taking itself off the list, invalidagin
the semaphore. */
sys_sem_signal(&scb->sem);
}
}
last_select_cb_ctr = select_cb_ctr;
/* unlock interrupts with each step */
SYS_ARCH_UNPROTECT(lev);
/* this makes sure interrupt protection time is short */
SYS_ARCH_PROTECT(lev);
if (last_select_cb_ctr != select_cb_ctr) {
/* someone has changed select_cb_list, restart at the beginning */
goto again;
}
Thanks,
Joel
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