Imbus/xv6-riscv-kernel
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use acquire/release to force order for pid=np->pid;np->state=RUNNING

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for bug reported by symingz@gmail.com and cs1100254@cse.iitd.ernet.in
This commit is contained in:
Robert Morris 2014-08-04 06:13:49 -04:00
parent 86188d9d49
commit 020c8e2384
3 changed files with 18 additions and 10 deletions
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@ -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;".
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.
This works because proc.h marks the pid as volatile.
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.