diff options
| -rw-r--r-- | TRICKS | 24 | ||||
| -rw-r--r-- | sysproc.c | 4 |
2 files changed, 27 insertions, 1 deletions
@@ -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;". + @@ -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 |