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rsc 2007-11-28 20:47:22 +00:00
parent 4f06ae0d42
commit c2258bf4d2
2 changed files with 27 additions and 1 deletions
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24
TRICKS
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@ -110,3 +110,27 @@ moves reads down after writes, but the language in
the spec allows it. There is no telling whether future
processors will need it.
---
The code in sys_fork needs to read np->pid before
setting np->state to RUNNABLE.
int
sys_fork(void)
{
int pid;
struct proc *np;
if((np = copyproc(cp)) == 0)
return -1;
pid = np->pid;
np->state = RUNNABLE;
return pid;
}
After setting np->state to RUNNABLE, some other CPU
might run the process, it might exit, and then it might
get reused for a different process (with a new pid), all
before the return statement. So it's not safe to just do
"return np->pid;".

4
sysproc.c
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@ -7,12 +7,14 @@
int
sys_fork(void)
{
int pid;
struct proc *np;
if((np = copyproc(cp)) == 0)
return -1;
pid = np->pid;
np->state = RUNNABLE;
return np->pid;
return pid;
}
int