Messages in this thread | | | Subject | Re: [RFC][PATCH] Fix a race between rwsem and the scheduler | From | Balbir Singh <> | Date | Wed, 31 Aug 2016 13:25:28 +1000 |
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On 30/08/16 22:58, Oleg Nesterov wrote: > On 08/30, Balbir Singh wrote: >> >> The origin of the issue I've seen seems to be related to >> rwsem spin lock stealing. Basically I see the system deadlock'd in the >> following state >> >> I have a system with multiple threads and >> >> Most of the threads are stuck doing >> >> [67272.593915] --- interrupt: e81 at _raw_spin_lock_irqsave+0xa4/0x130 >> [67272.593915] LR = _raw_spin_lock_irqsave+0x9c/0x130 >> [67272.700996] [c000000012857ae0] [c00000000012453c] rwsem_wake+0xcc/0x110 >> [67272.749283] [c000000012857b20] [c0000000001215d8] up_write+0x78/0x90 >> [67272.788965] [c000000012857b50] [c00000000028153c] unlink_anon_vmas+0x15c/0x2c0 >> [67272.798782] [c000000012857bc0] [c00000000026f5c0] free_pgtables+0xf0/0x1c0 >> [67272.842528] [c000000012857c10] [c00000000027c9a0] exit_mmap+0x100/0x1a0 >> [67272.872947] [c000000012857cd0] [c0000000000b4a98] mmput+0xa8/0x1b0 >> [67272.898432] [c000000012857d00] [c0000000000bc50c] do_exit+0x33c/0xc30 >> [67272.944721] [c000000012857dc0] [c0000000000bcee4] do_group_exit+0x64/0x100 >> [67272.969014] [c000000012857e00] [c0000000000bcfac] SyS_exit_group+0x2c/0x30 >> [67272.978971] [c000000012857e30] [c000000000009204] system_call+0x38/0xb4 >> [67272.999016] Instruction dump: >> >> They are spinning on the sem->wait_lock, the holder of sem->wait_lock has >> irq's disabled and is doing >> >> [c00000037930fb30] c0000000000f724c try_to_wake_up+0x6c/0x570 >> [c00000037930fbb0] c000000000124328 __rwsem_do_wake+0x1f8/0x260 >> [c00000037930fc00] c0000000001244b4 rwsem_wake+0x84/0x110 >> [c00000037930fc40] c000000000121598 up_write+0x78/0x90 >> [c00000037930fc70] c000000000281a54 anon_vma_fork+0x184/0x1d0 >> [c00000037930fcc0] c0000000000b68e0 copy_process.isra.5+0x14c0/0x1870 >> [c00000037930fda0] c0000000000b6e68 _do_fork+0xa8/0x4b0 >> [c00000037930fe30] c000000000009460 ppc_clone+0x8/0xc >> >> The offset of try_to_wake_up is actually misleading, it is actually stuck >> doing the following in try_to_wake_up >> >> while (p->on_cpu) >> cpu_relax(); >> >> Analysis >> >> The issue is triggered, due to the following race >> >> CPU1 CPU2 >> >> while () { >> if (cond) >> break; >> do { >> schedule(); >> set_current_state(TASK_UN..) >> } while (!cond); >> rwsem_wake() >> spin_lock_irqsave(wait_lock) >> raw_spin_lock_irqsave(wait_lock) wake_up_process() >> } try_to_wake_up() >> set_current_state(TASK_RUNNING); .. >> list_del(&waiter.list); >> >> CPU2 wakes up CPU1, but before it can get the wait_lock and set >> current state to TASK_RUNNING the following occurs >> >> CPU3 >> (stole the rwsem before waiter can be woken up from queue) >> up_write() >> rwsem_wake() >> raw_spin_lock_irqsave(wait_lock) >> if (!list_empty) >> wake_up_process() >> try_to_wake_up() >> raw_spin_lock_irqsave(p->pi_lock) >> .. >> if (p->on_rq && ttwu_wakeup()) >> .. >> while (p->on_cpu) >> cpu_relax() >> .. >> >> CPU3 tries to wake up the task on CPU1 again since it finds >> it on the wait_queue, CPU1 is spinning on wait_lock, but immediately >> after CPU2, CPU3 got it. >> >> CPU3 checks the state of p on CPU1, it is TASK_UNINTERRUPTIBLE and >> the task is spinning on the wait_lock. Interestingly since p->on_rq >> is checked under pi_lock, I've noticed that try_to_wake_up() finds >> p->on_rq to be 0. This was the most confusing bit of the analysis, >> but p->on_rq is changed under runqueue lock, rq_lock, the p->on_rq >> check is not reliable without this fix IMHO. The race is visible >> (based on the analysis) only when ttwu_queue() does a remote wakeup >> via ttwu_queue_remote. In which case the p->on_rq change is not >> done uder the pi_lock. >> >> The result is that after a while the entire system locks up on >> the raw_spin_irqlock_save(wait_lock) and the holder spins infintely >> >> Reproduction of the issue >> >> The issue can be reproduced after a long run on my system with 80 >> threads and having to tweak available memory to very low and running >> memory stress-ng mmapfork test. It usually takes a long time to >> reproduce. I am trying to work on a test case that can reproduce >> the issue faster, but thats work in progress. I am still testing the >> changes on my still in a loop and the tests seem OK thus far. >> >> Big thanks to Benjamin and Nick for helping debug this as well. >> Ben helped catch the missing barrier, Nick caught every missing >> bit in my theory >> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Nicholas Piggin <npiggin@gmail.com> >> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> >> >> Signed-off-by: Balbir Singh <bsingharora@gmail.com> >> --- >> kernel/sched/core.c | 11 +++++++++++ >> 1 file changed, 11 insertions(+) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 2a906f2..582c684 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2016,6 +2016,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) >> success = 1; /* we're going to change ->state */ >> cpu = task_cpu(p); >> >> + /* >> + * Ensure we see on_rq and p_state consistently >> + * >> + * For example in __rwsem_down_write_failed(), we have >> + * [S] ->on_rq = 1 [L] ->state >> + * MB RMB >> + * [S] ->state = TASK_UNINTERRUPTIBLE [L] ->on_rq >> + * In the absence of the RMB p->on_rq can be observed to be 0 >> + * and we end up spinning indefinitely in while (p->on_cpu) >> + */ >> + smp_rmb(); > > I think the patch is fine... but unless I am totally confused this > is not specific to __rwsem_down_write_failed(). ttwu() can hang the > same way if the target simply does > > schedule_timeout(); > current->state = TASK_INTERRUPTIBLE;
Yes
> current->state = TASK_RUNNING; > > And. I am not sure I understand where this MB above comes from.
I think Ben pointed that out earlier and you found it in the documentation as well at the end of __switch
Balbir Singh
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