Messages in this thread | | | Date | Wed, 4 Apr 2007 22:55:01 -0700 | From | "Paul Menage" <> | Subject | Re: [PATCH] Fix race between attach_task and cpuset_exit |
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On 3/26/07, Srivatsa Vaddagiri <vatsa@in.ibm.com> wrote: > On Sun, Mar 25, 2007 at 12:50:25PM -0700, Paul Jackson wrote: > > Is there perhaps another race here? > > Yes, we have! > > Modified patch below. Compile/boot tested on a x86_64 box. > > > Currently cpuset_exit() changes the exiting task's ->cpuset pointer w/o > taking task_lock(). This can lead to ugly races between attach_task and > cpuset_exit. Details of the races are described at > http://lkml.org/lkml/2007/3/24/132. > > Patch below closes those races. It is against 2.6.21-rc4 and has > undergone a simple compile/boot test on a x86_64 box. > > Signed-off-by : Srivatsa Vaddagiri <vatsa@in.ibm.com> > > > --- > > > diff -puN kernel/cpuset.c~cpuset_race_fix kernel/cpuset.c > --- linux-2.6.21-rc4/kernel/cpuset.c~cpuset_race_fix 2007-03-25 21:08:27.000000000 +0530 > +++ linux-2.6.21-rc4-vatsa/kernel/cpuset.c 2007-03-26 16:48:24.000000000 +0530 > @@ -1182,6 +1182,7 @@ static int attach_task(struct cpuset *cs > pid_t pid; > struct task_struct *tsk; > struct cpuset *oldcs; > + struct cpuset *oldcs_to_be_released = NULL; > cpumask_t cpus; > nodemask_t from, to; > struct mm_struct *mm; > @@ -1237,6 +1238,8 @@ static int attach_task(struct cpuset *cs > } > atomic_inc(&cs->count); > rcu_assign_pointer(tsk->cpuset, cs); > + if (atomic_dec_and_test(&oldcs->count)) > + oldcs_to_be_released = oldcs; > task_unlock(tsk); > > guarantee_online_cpus(cs, &cpus); > @@ -1257,8 +1260,8 @@ static int attach_task(struct cpuset *cs > > put_task_struct(tsk); > synchronize_rcu(); > - if (atomic_dec_and_test(&oldcs->count)) > - check_for_release(oldcs, ppathbuf); > + if (oldcs_to_be_released) > + check_for_release(oldcs_to_be_released, ppathbuf); > return 0; > }
Is this part of the patch necessary? If we're adding a task_lock() in cpuset_exit(), then the problem that Vatsa described (both cpuset_attach_task() and cpuset_exit() decrementing the same cpuset count, and cpuset_attach_task() incrementing the count on a cpuset that the task doesn't eventually end up in) go away, since only one thread will retrieve the old value of the task's cpuset in order to decrement its count.
> > @@ -2200,10 +2203,6 @@ void cpuset_fork(struct task_struct *chi > * it is holding that mutex while calling check_for_release(), > * which calls kmalloc(), so can't be called holding callback_mutex(). > * > - * We don't need to task_lock() this reference to tsk->cpuset, > - * because tsk is already marked PF_EXITING, so attach_task() won't > - * mess with it, or task is a failed fork, never visible to attach_task. > - * > * the_top_cpuset_hack: > * > * Set the exiting tasks cpuset to the root cpuset (top_cpuset). > @@ -2241,20 +2240,23 @@ void cpuset_fork(struct task_struct *chi > void cpuset_exit(struct task_struct *tsk) > { > struct cpuset *cs; > + struct cpuset *oldcs_to_be_released = NULL; > > + task_lock(tsk); > cs = tsk->cpuset; > tsk->cpuset = &top_cpuset; /* the_top_cpuset_hack - see above */ > + if (atomic_dec_and_test(&cs->count)) > + oldcs_to_be_released = cs; > + task_unlock(tsk); >
I think this is still racy - at this point we're holding a reference on a cpuset that could have a zero count, and we don't hold manage_mutex or callback_mutex. So a concurrent rmdir could zap the directory and free the cpuset.
Shouldn't we just put a task_lock()/task_unlock() around these lines and leave everything else as-is?
task_lock(tsk); cs = tsk->cpuset; tsk->cpuset = &top_cpuset; /* the_top_cpuset_hack - see above */ task_unlock(tsk)
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