diff options
Diffstat (limited to 'TRICKS')
| -rw-r--r-- | TRICKS | 18 |
1 files changed, 11 insertions, 7 deletions
@@ -116,21 +116,25 @@ processors will need it. --- The code in fork needs to read np->pid before -setting np->state to RUNNABLE. +setting np->state to RUNNABLE. The following +is not a correct way to do this: int fork(void) { ... - pid = np->pid; np->state = RUNNABLE; - return pid; + return np->pid; // oops } 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;". - -This works because proc.h marks the pid as volatile. +before the return statement. So it's not safe to just +"return np->pid". Even saving a copy of np->pid before +setting np->state isn't safe, since the compiler is +allowed to re-order statements. + +The real code saves a copy of np->pid, then acquires a lock +around the write to np->state. The acquire() prevents the +compiler from re-ordering. |