The get_pid function in kernel/fork.c returns the value of last_pid
after releasing the lock protecting it. On SMPs, I have observed two
processes occasionally being assigned the same process ID as a result.
I'm pretty sure the sequence of events looks like this: The first
process releases the lock after coming up with a suitable pid (stored
in last_pid). Then after it releases the lock but before it does the
return (IRQ or something happens here to create delay?) another
processor comes a long and updates it. Then get_pid returns the value
of last_pid which is now the pid chosen by the other process.
Attached below is a little patch to fix it.
--- linux-2.4.17/kernel/fork.c.orig Mon Feb 4 14:53:31 2002
static int next_safe = PID_MAX;
struct task_struct *p;
+ int pid;
if (flags & CLONE_PID)
+ pid = last_pid;
- return last_pid;
+ return pid;
static inline int dup_mmap(struct mm_struct * mm)
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