Paul Jackson writes:
> The cpusets-formalize-intermediate-gfp_kernel-containment patch
> has a deadlock problem.
[...]
>
> /*
> + * The global cpuset semaphore cpuset_sem can be needed by the
> + * memory allocator to update a tasks mems_allowed (see the calls
> + * to cpuset_update_current_mems_allowed()) or to walk up the
> + * cpuset hierarchy to find a mem_exclusive cpuset see the calls
> + * to cpuset_excl_nodes_overlap()).
> + *
> + * But if the memory allocation is being done by cpuset.c code, it
> + * usually already holds cpuset_sem. Double tripping on a kernel
> + * semaphore deadlocks the current task, and any other task that
> + * subsequently tries to obtain the lock.
> + *
> + * Run all up's and down's on cpuset_sem through the following
> + * wrappers, which will detect this nested locking, and avoid
> + * deadlocking.
> + */
> +
> +static inline void cs_down(struct semaphore *psem)
> +{
> + if (current->cpuset_sem_nest_depth == 0)
> + down(psem);
> + current->cpuset_sem_nest_depth++;
> +}
> +
> +static inline void cs_up(struct semaphore *psem)
> +{
> + current->cpuset_sem_nest_depth--;
> + if (current->cpuset_sem_nest_depth == 0)
> + up(psem);
> +}
I am somewhat concerned that new fields are added to the struct
task_struct all the time: it's already over 1.3KB.
In that particular case, it seems that cs_{up,down}() (or however they
end up being named), are used only on &cpuset_sem, and adding new field
to the thread struct can be avoided by doing:
static DECLARE_MUTEX(cpuset_sem);
static struct task_struct *cpuset_sem_owner = NULL;
static int cpuset_sem_depth = 0;
static void cpusets_lock(void)
{
if (cpuset_sem_owner != current) {
down(&cpuset_sem);
cpuset_sem_owner = current;
}
cpuset_sem_depth ++;
}
static void cpusets_unlock(void)
{
if (-- cpuset_sem_depth == 0) {
cpuset_sem_owner = NULL;
up(&cpuset_sem);
}
}
Nikita.
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