From d69b78ba1deaaa95ffa8dac5a9ca819ce454d52e Mon Sep 17 00:00:00 2001 From: Greg Thelen Date: Mon, 15 Nov 2010 10:20:52 +0100 Subject: [PATCH] ioprio: grab rcu_read_lock in sys_ioprio_{set,get}() Using: - CONFIG_LOCKUP_DETECTOR=y - CONFIG_PREEMPT=y - CONFIG_LOCKDEP=y - CONFIG_PROVE_LOCKING=y - CONFIG_PROVE_RCU=y found a missing rcu lock during boot on a 512 MiB x86_64 ubuntu vm: =================================================== [ INFO: suspicious rcu_dereference_check() usage. ] --------------------------------------------------- kernel/pid.c:419 invoked rcu_dereference_check() without protection! other info that might help us debug this: rcu_scheduler_active = 1, debug_locks = 0 1 lock held by ureadahead/1355: #0: (tasklist_lock){.+.+..}, at: [] sys_ioprio_set+0x7f/0x29e stack backtrace: Pid: 1355, comm: ureadahead Not tainted 2.6.37-dbg-DEV #1 Call Trace: [] lockdep_rcu_dereference+0xaa/0xb3 [] find_task_by_pid_ns+0x44/0x5d [] find_task_by_vpid+0x22/0x24 [] sys_ioprio_set+0xb4/0x29e [] ? trace_hardirqs_off_thunk+0x3a/0x3c [] sysenter_dispatch+0x7/0x2c [] ? trace_hardirqs_on_thunk+0x3a/0x3f The fix is to: a) grab rcu lock in sys_ioprio_{set,get}() and b) avoid grabbing tasklist_lock. Discussion in: http://marc.info/?l=linux-kernel&m=128951324702889 Signed-off-by: Greg Thelen Acked-by: Paul E. McKenney Reviewed-by: Oleg Nesterov Modified by Jens to remove the now redundant inner rcu lock and unlock since they are now protected by the outer lock. Signed-off-by: Jens Axboe --- fs/ioprio.c | 31 ++++++------------------------- 1 file changed, 6 insertions(+), 25 deletions(-) diff --git a/fs/ioprio.c b/fs/ioprio.c index 2f7d05c89922..7da2a06508e5 100644 --- a/fs/ioprio.c +++ b/fs/ioprio.c @@ -103,22 +103,15 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio) } ret = -ESRCH; - /* - * We want IOPRIO_WHO_PGRP/IOPRIO_WHO_USER to be "atomic", - * so we can't use rcu_read_lock(). See re-copy of ->ioprio - * in copy_process(). - */ - read_lock(&tasklist_lock); + rcu_read_lock(); switch (which) { case IOPRIO_WHO_PROCESS: - rcu_read_lock(); if (!who) p = current; else p = find_task_by_vpid(who); if (p) ret = set_task_ioprio(p, ioprio); - rcu_read_unlock(); break; case IOPRIO_WHO_PGRP: if (!who) @@ -141,12 +134,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio) break; do_each_thread(g, p) { - int match; - - rcu_read_lock(); - match = __task_cred(p)->uid == who; - rcu_read_unlock(); - if (!match) + if (__task_cred(p)->uid != who) continue; ret = set_task_ioprio(p, ioprio); if (ret) @@ -160,7 +148,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio) ret = -EINVAL; } - read_unlock(&tasklist_lock); + rcu_read_unlock(); return ret; } @@ -204,17 +192,15 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who) int ret = -ESRCH; int tmpio; - read_lock(&tasklist_lock); + rcu_read_lock(); switch (which) { case IOPRIO_WHO_PROCESS: - rcu_read_lock(); if (!who) p = current; else p = find_task_by_vpid(who); if (p) ret = get_task_ioprio(p); - rcu_read_unlock(); break; case IOPRIO_WHO_PGRP: if (!who) @@ -241,12 +227,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who) break; do_each_thread(g, p) { - int match; - - rcu_read_lock(); - match = __task_cred(p)->uid == user->uid; - rcu_read_unlock(); - if (!match) + if (__task_cred(p)->uid != user->uid) continue; tmpio = get_task_ioprio(p); if (tmpio < 0) @@ -264,6 +245,6 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who) ret = -EINVAL; } - read_unlock(&tasklist_lock); + rcu_read_unlock(); return ret; }