Re: [PATCH] block: Fix sys_ioprio_set(.which=IOPRIO_WHO_PGRP) task iteration

[Oleg Nesterov]

On 04/08, Jens Axboe wrote:
>
> On 4/8/21 3:46 AM, Peter Zijlstra wrote:
> >
> > do_each_pid_thread() { } while_each_pid_thread() is a double loop and
> > thus break doesn't work as expected. Also, it should be used under
> > tasklist_lock because otherwise we can race against change_pid() for
> > PGID/SID.
>
> Applied, thanks.

Agreed, but can't resist. We can move the "out" label up and avoid the extra
read_unlock(tasklist_lock). IOW, something like below on top of this patch.

Quite possibly this won't change the generated code, gcc is smart enough, but
this makes the code a bit more readable.

Oleg.

--- x/block/ioprio.c~   2021-04-09 12:00:28.066145563 +0200
+++ x/block/ioprio.c2021-04-09 12:02:01.817849618 +0200
@@ -123,11 +123,10 @@
read_lock(_lock);
do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
ret = set_task_ioprio(p, ioprio);
-   if (ret) {
-   read_unlock(_lock);
-   goto out;
-   }
+   if (ret)
+   goto out_pgrp;
} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+out_pgrp:
read_unlock(_lock);
 
break;
@@ -159,7 +158,6 @@
ret = -EINVAL;
}
 
-out:
rcu_read_unlock();
return ret;
 }

Re: [PATCH] block: Fix sys_ioprio_set(.which=IOPRIO_WHO_PGRP) task iteration

[Jens Axboe]

On 4/8/21 3:46 AM, Peter Zijlstra wrote:
> 
> do_each_pid_thread() { } while_each_pid_thread() is a double loop and
> thus break doesn't work as expected. Also, it should be used under
> tasklist_lock because otherwise we can race against change_pid() for
> PGID/SID.

Applied, thanks.

-- 
Jens Axboe

[PATCH] block: Fix sys_ioprio_set(.which=IOPRIO_WHO_PGRP) task iteration

[Peter Zijlstra]

do_each_pid_thread() { } while_each_pid_thread() is a double loop and
thus break doesn't work as expected. Also, it should be used under
tasklist_lock because otherwise we can race against change_pid() for
PGID/SID.

Signed-off-by: Peter Zijlstra (Intel) 
---
 block/ioprio.c |   11 +--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -119,11 +119,17 @@ SYSCALL_DEFINE3(ioprio_set, int, which,
pgrp = task_pgrp(current);
else
pgrp = find_vpid(who);
+
+   read_lock(_lock);
do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
ret = set_task_ioprio(p, ioprio);
-   if (ret)
-   break;
+   if (ret) {
+   read_unlock(_lock);
+   goto out;
+   }
} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+   read_unlock(_lock);
+
break;
case IOPRIO_WHO_USER:
uid = make_kuid(current_user_ns(), who);
@@ -153,6 +159,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which,
ret = -EINVAL;
}
 
+out:
rcu_read_unlock();
return ret;
 }