From a69ac4a78d8bd9e1ec478bd7297d4f047fcd44a8 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov 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 Signed-off-by: Linus Torvalds --- 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 92a038064628fe..b15462b17a5827 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; }