I am trying to understand the locking in net/socket.c.
Suppose the system is uniprocessor (no preemption).
Then the various locking routines do nothing:
#define net_family_write_lock() do { } while(0)
#define net_family_write_unlock() do { } while(0)
#define net_family_read_lock() do { } while(0)
#define net_family_read_unlock() do { } while(0)
Look in sock_create:
net_family_read_lock();
...
if ((i = net_families[family]->create(sock, protocol)) < 0) <= may sleep
...
net_family_read_unlock();
The call to create(...) may sleep. Suppose during this sleep a task is
run that calls sock_unregister:
int sock_unregister(int family)
{
if (family < 0 || family >= NPROTO)
return -1;
net_family_write_lock();
net_families[family]=NULL;
net_family_write_unlock();
return 0;
}
Since net_family_write_lock() is a noop, this succeeds, and returns.
Happy is my task! It has returned from sock_unregister, so can now,
for example, free memory used for implementing that protocol. But the
original create(...) call is in the middle of using that protocol. Unhappy
am I! I am dead!
What have I missed?
Ciao, Duncan.
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