chiark / gitweb /
poll: Support deregistration from the main event loop
authorIan Jackson <ijackson@chiark.greenend.org.uk>
Sat, 20 Sep 2014 12:56:04 +0000 (13:56 +0100)
committerIan Jackson <ijackson@chiark.greenend.org.uk>
Mon, 6 Oct 2014 22:29:24 +0000 (23:29 +0100)
The logic here is slightly subtle because of reentrancy hazards.  See
the comment in deregister_for_poll.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
secnet.c
secnet.h

index 4520b3e..111a1b0 100644 (file)
--- a/secnet.c
+++ b/secnet.c
@@ -36,7 +36,7 @@ static pid_t secnet_pid;
 
 /* Structures dealing with poll() call */
 struct poll_interest {
-    beforepoll_fn *before;
+    beforepoll_fn *before; /* 0 if deregistered and waiting to be deleted */
     afterpoll_fn *after;
     void *state;
     int32_t nfds;
@@ -45,6 +45,11 @@ struct poll_interest {
 };
 static LIST_HEAD(, poll_interest) reg = LIST_HEAD_INITIALIZER(&reg);
 
+static bool_t interest_isregistered(const struct poll_interest *i)
+{
+    return !!i->before;
+}
+
 static bool_t finished=False;
 
 /* Parse the command line options */
@@ -225,7 +230,7 @@ static void setup(dict_t *config)
     }
 }
 
-void register_for_poll(void *st, beforepoll_fn *before,
+struct poll_interest *register_for_poll(void *st, beforepoll_fn *before,
                       afterpoll_fn *after, cstring_t desc)
 {
     struct poll_interest *i;
@@ -237,7 +242,15 @@ void register_for_poll(void *st, beforepoll_fn *before,
     i->nfds=0;
     i->desc=desc;
     LIST_INSERT_HEAD(&reg, i, entry);
-    return;
+    return i;
+}
+
+void deregister_for_poll(struct poll_interest *i)
+{
+    /* We cannot simply throw this away because we're reentrantly
+     * inside the main loop, which needs to remember which range of
+     * fds corresponds to this now-obsolete interest */
+    i->before=0;
 }
 
 static void system_phase_hook(void *sst, uint32_t newphase)
@@ -288,7 +301,7 @@ uint64_t now_global;
 
 static void run(void)
 {
-    struct poll_interest *i;
+    struct poll_interest *i, *itmp;
     int rv, nfds, idx;
     int timeout;
     struct pollfd *fds=0;
@@ -305,12 +318,14 @@ static void run(void)
        idx=0;
        LIST_FOREACH(i, &reg, entry) {
            int check;
-           for (check=0; check<i->nfds; check++) {
-               if(fds[idx+check].revents & POLLNVAL) {
-                   fatal("run: poll (%s#%d) set POLLNVAL", i->desc, check);
+           if (interest_isregistered(i)) {
+               for (check=0; check<i->nfds; check++) {
+                   if(fds[idx+check].revents & POLLNVAL) {
+                       fatal("run: poll (%s#%d) set POLLNVAL", i->desc, check);
+                   }
                }
+               i->after(i->state, fds+idx, i->nfds);
            }
-           i->after(i->state, fds+idx, i->nfds);
            idx+=i->nfds;
        }
        if (shortfall) {
@@ -321,22 +336,33 @@ static void run(void)
        shortfall=0;
        idx=0;
        timeout=-1;
-       LIST_FOREACH(i, &reg, entry) {
+       LIST_FOREACH_SAFE(i, &reg, entry, itmp) {
            int remain=allocdfds-idx;
            nfds=remain;
-           rv=i->before(i->state, fds+idx, &nfds, &timeout);
-           if (rv!=0) {
-               if (rv!=ERANGE)
-                   fatal("run: beforepoll_fn (%s) returns %d",i->desc,rv);
-               assert(nfds < INT_MAX/4 - shortfall);
-               shortfall += nfds-remain;
+           if (interest_isregistered(i)) {
+               rv=i->before(i->state, fds+idx, &nfds, &timeout);
+               if (rv!=0) {
+                   if (rv!=ERANGE)
+                       fatal("run: beforepoll_fn (%s) returns %d",i->desc,rv);
+                   assert(nfds < INT_MAX/4 - shortfall);
+                   shortfall += nfds-remain;
+                   nfds=0;
+                   timeout=0;
+               }
+           } else {
                nfds=0;
-               timeout=0;
            }
            if (timeout<-1) {
                fatal("run: beforepoll_fn (%s) set timeout to %d",
                      i->desc,timeout);
            }
+           if (!interest_isregistered(i)) {
+               /* check this here, rather than earlier, so that we
+                  handle the case where i->before() calls deregister */
+               LIST_REMOVE(i, entry);
+               free(i);
+               continue;
+           }
            idx+=nfds;
            i->nfds=nfds;
        }
index 1e1e75c..b5028bb 100644 (file)
--- a/secnet.h
+++ b/secnet.h
@@ -216,8 +216,9 @@ typedef void afterpoll_fn(void *st, struct pollfd *fds, int nfds);
 /* Register interest in the main loop of the program. Before a call
    to poll() your supplied beforepoll function will be called. After
    the call to poll() the supplied afterpoll function will be called. */
-extern void register_for_poll(void *st, beforepoll_fn *before,
+struct poll_interest *register_for_poll(void *st, beforepoll_fn *before,
                              afterpoll_fn *after, cstring_t desc);
+void deregister_for_poll(struct poll_interest *i);
 
 /***** END of scheduling support */