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

[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) <peterz@infradead.org>
---
 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(&tasklist_lock);
 			do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
 				ret = set_task_ioprio(p, ioprio);
-				if (ret)
-					break;
+				if (ret) {
+					read_unlock(&tasklist_lock);
+					goto out;
+				}
 			} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+			read_unlock(&tasklist_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;
 }

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

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