On 11/26/2013 03:03 PM, Peter Zijlstra wrote:
> Juri hit the below lockdep report:
>
> [ 4.303391] ======================================================
> [ 4.303392] [ INFO: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected ]
> [ 4.303394] 3.12.0-dl-peterz+ #144 Not tainted
> [ 4.303395] ------------------------------------------------------
> [ 4.303397] kworker/u4:3/689 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> [ 4.303399] (&p->mems_allowed_seq){+.+...}, at: [<ffffffff8114e63c>] new_slab+0x6c/0x290
> [ 4.303417]
> [ 4.303417] and this task is already holding:
> [ 4.303418] (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffff812d2dfb>] blk_execute_rq_nowait+0x5b/0x100
> [ 4.303431] which would create a new lock dependency:
> [ 4.303432] (&(&q->__queue_lock)->rlock){..-...} -> (&p->mems_allowed_seq){+.+...}
> [ 4.303436]
>
> [ 4.303898] the dependencies between the lock to be acquired and SOFTIRQ-irq-unsafe lock:
> [ 4.303918] -> (&p->mems_allowed_seq){+.+...} ops: 2762 {
> [ 4.303922] HARDIRQ-ON-W at:
> [ 4.303923] [<ffffffff8108ab9a>] __lock_acquire+0x65a/0x1ff0
> [ 4.303926] [<ffffffff8108cbe3>] lock_acquire+0x93/0x140
> [ 4.303929] [<ffffffff81063dd6>] kthreadd+0x86/0x180
> [ 4.303931] [<ffffffff816ded6c>] ret_from_fork+0x7c/0xb0
> [ 4.303933] SOFTIRQ-ON-W at:
> [ 4.303933] [<ffffffff8108abcc>] __lock_acquire+0x68c/0x1ff0
> [ 4.303935] [<ffffffff8108cbe3>] lock_acquire+0x93/0x140
> [ 4.303940] [<ffffffff81063dd6>] kthreadd+0x86/0x180
> [ 4.303955] [<ffffffff816ded6c>] ret_from_fork+0x7c/0xb0
> [ 4.303959] INITIAL USE at:
> [ 4.303960] [<ffffffff8108a884>] __lock_acquire+0x344/0x1ff0
> [ 4.303963] [<ffffffff8108cbe3>] lock_acquire+0x93/0x140
> [ 4.303966] [<ffffffff81063dd6>] kthreadd+0x86/0x180
> [ 4.303969] [<ffffffff816ded6c>] ret_from_fork+0x7c/0xb0
> [ 4.303972] }
>
> Which reports that we take mems_allowed_seq with interrupts enabled. A
> little digging found that this can only be from
> cpuset_change_task_nodemask().
>
> This is an actual deadlock because an interrupt doing an allocation will
> hit get_mems_allowed()->...->__read_seqcount_begin(), which will spin
> forever waiting for the write side to complete.
>
And this patch fixes it, thanks!
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Reported-by: Juri Lelli <juri.lelli@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Tested-by: Juri Lelli <juri.lelli@xxxxxxxxx>
Best,
- Juri
> ---
> kernel/cpuset.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index 6bf981e13c43..4772034b4b17 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -1033,8 +1033,10 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
> need_loop = task_has_mempolicy(tsk) ||
> !nodes_intersects(*newmems, tsk->mems_allowed);
>
> - if (need_loop)
> + if (need_loop) {
> + local_irq_disable();
> write_seqcount_begin(&tsk->mems_allowed_seq);
> + }
>
> nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
> mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1);
> @@ -1042,8 +1044,10 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
> mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP2);
> tsk->mems_allowed = *newmems;
>
> - if (need_loop)
> + if (need_loop) {
> write_seqcount_end(&tsk->mems_allowed_seq);
> + local_irq_enable();
> + }
>
> task_unlock(tsk);
> }
>
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