From a69ac4a78d8bd9e1ec478bd7297d4f047fcd44a8 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@tv-sign.ru>
Date: Mon, 24 Oct 2005 18:29:58 +0400
Subject: [PATCH] [PATCH] posix-timers: fix posix_cpu_timer_set() vs
run_posix_cpu_timers() race
This might be harmless, but looks like a race from code inspection (I
was unable to trigger it). I must admit, I don't understand why we
can't return TIMER_RETRY after 'spin_unlock(&p->sighand->siglock)'
without doing bump_cpu_timer(), but this is what original code does.
posix_cpu_timer_set:
read_lock(&tasklist_lock);
spin_lock(&p->sighand->siglock);
list_del_init(&timer->it.cpu.entry);
spin_unlock(&p->sighand->siglock);
We are probaly deleting the timer from run_posix_cpu_timers's 'firing'
local list_head while run_posix_cpu_timers() does list_for_each_safe.
Various bad things can happen, for example we can just delete this timer
so that list_for_each() will not notice it and run_posix_cpu_timers()
will not reset '->firing' flag. In that case,
....
if (timer->it.cpu.firing) {
read_unlock(&tasklist_lock);
timer->it.cpu.firing = -1;
return TIMER_RETRY;
}
sys_timer_settime() goes to 'retry:', calls posix_cpu_timer_set() again,
it returns TIMER_RETRY ...
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---
kernel/posix-cpu-timers.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 92a038064628..b15462b17a58 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -730,9 +730,15 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
* Disarm any old timer after extracting its expiry time.
*/
BUG_ON(!irqs_disabled());
+
+ ret = 0;
spin_lock(&p->sighand->siglock);
old_expires = timer->it.cpu.expires;
- list_del_init(&timer->it.cpu.entry);
+ if (unlikely(timer->it.cpu.firing)) {
+ timer->it.cpu.firing = -1;
+ ret = TIMER_RETRY;
+ } else
+ list_del_init(&timer->it.cpu.entry);
spin_unlock(&p->sighand->siglock);
/*
@@ -780,7 +786,7 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
}
}
- if (unlikely(timer->it.cpu.firing)) {
+ if (unlikely(ret)) {
/*
* We are colliding with the timer actually firing.
* Punt after filling in the timer's old value, and
@@ -788,8 +794,6 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
* it as an overrun (thanks to bump_cpu_timer above).
*/
read_unlock(&tasklist_lock);
- timer->it.cpu.firing = -1;
- ret = TIMER_RETRY;
goto out;
}
--
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