Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751576AbdLYCUn (ORCPT ); Sun, 24 Dec 2017 21:20:43 -0500 Received: from mail5.windriver.com ([192.103.53.11]:33086 "EHLO mail5.wrs.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751112AbdLYCUk (ORCPT ); Sun, 24 Dec 2017 21:20:40 -0500 From: Qi Hou To: , CC: , , , , , Subject: [RFC] dmaengine: pl330: fix a race condition in case of threaded irqs Date: Mon, 25 Dec 2017 10:20:16 +0800 Message-ID: <1514168416-4285-1-git-send-email-qi.hou@windriver.com> X-Mailer: git-send-email 2.7.4 MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [128.224.162.179] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7958 Lines: 176 I found this problem below, and I now understand why it happens, but I'm not 100% sure what is the best way to fix it. When booting up with "threadirqs" in command line, all irq handlers of the DMA controller pl330 will be threaded forcedly. These threads will race for the same list, pl330->req_done. Before the callback, the spinlock was released. And after it, the spinlock was taken. This opened an race window where another threaded irq handler could steal the spinlock and be permitted to delete entries of the list, pl330->req_done. If the later deleted an entry that was still referred to by the former, there would be a kernel panic when the former was scheduled and tried to get the next sibling of the deleted entry. The scenario could be depicted as below: Thread: T1 pl330->req_done Thread: T2 | | | | -A-B-C-D- | Locked | | | | Waiting Del A | | | -B-C-D- | Unlocked | | | | Locked Waiting | | | | Del B | | | | -C-D- Unlocked Waiting | | | Locked | get C via B \ - Kernel panic The kernel panic looked like as below: Unable to handle kernel paging request at virtual address dead000000000108 pgd = ffffff8008c9e000 [dead000000000108] *pgd=000000027fffe003, *pud=000000027fffe003, *pmd=0000000000000000 Internal error: Oops: 96000044 [#1] PREEMPT SMP Modules linked in: CPU: 0 PID: 85 Comm: irq/59-66330000 Not tainted 4.8.24-WR9.0.0.12_standard #2 Hardware name: Broadcom NS2 SVK (DT) task: ffffffc1f5cc3c00 task.stack: ffffffc1f5ce0000 PC is at pl330_irq_handler+0x27c/0x390 LR is at pl330_irq_handler+0x2a8/0x390 pc : [] lr : [] pstate: 800001c5 sp : ffffffc1f5ce3d00 x29: ffffffc1f5ce3d00 x28: 0000000000000140 x27: ffffffc1f5c530b0 x26: dead000000000100 x25: dead000000000200 x24: 0000000000418958 x23: 0000000000000001 x22: ffffffc1f5ccd668 x21: ffffffc1f5ccd590 x20: ffffffc1f5ccd418 x19: dead000000000060 x18: 0000000000000001 x17: 0000000000000007 x16: 0000000000000001 x15: ffffffffffffffff x14: ffffffffffffffff x13: ffffffffffffffff x12: 0000000000000000 x11: 0000000000000001 x10: 0000000000000840 x9 : ffffffc1f5ce0000 x8 : ffffffc1f5cc3338 x7 : ffffff8008ce2020 x6 : 0000000000000000 x5 : 0000000000000000 x4 : 0000000000000001 x3 : dead000000000200 x2 : dead000000000100 x1 : 0000000000000140 x0 : ffffffc1f5ccd590 Process irq/59-66330000 (pid: 85, stack limit = 0xffffffc1f5ce0020) Stack: (0xffffffc1f5ce3d00 to 0xffffffc1f5ce4000) 3d00: ffffffc1f5ce3d80 ffffff80080f09d0 ffffffc1f5ca0c00 ffffffc1f6f7c600 3d20: ffffffc1f5ce0000 ffffffc1f6f7c600 ffffffc1f5ca0c00 ffffff80080f0998 3d40: ffffffc1f5ce0000 ffffff80080f0000 0000000000000000 0000000000000000 3d60: ffffff8008ce202c ffffff8008ce2020 ffffffc1f5ccd668 ffffffc1f5c530b0 3d80: ffffffc1f5ce3db0 ffffff80080f0d70 ffffffc1f5ca0c40 0000000000000001 3da0: ffffffc1f5ce0000 ffffff80080f0cfc ffffffc1f5ce3e20 ffffff80080bf4f8 3dc0: ffffffc1f5ca0c80 ffffff8008bf3798 ffffff8008955528 ffffffc1f5ca0c00 3de0: ffffff80080f0c30 0000000000000000 0000000000000000 0000000000000000 3e00: 0000000000000000 0000000000000000 0000000000000000 ffffff80080f0b68 3e20: 0000000000000000 ffffff8008083690 ffffff80080bf420 ffffffc1f5ca0c80 3e40: 0000000000000000 0000000000000000 0000000000000000 ffffff80080cb648 3e60: ffffff8008b1c780 0000000000000000 0000000000000000 ffffffc1f5ca0c00 3e80: ffffffc100000000 ffffff8000000000 ffffffc1f5ce3e90 ffffffc1f5ce3e90 3ea0: 0000000000000000 ffffff8000000000 ffffffc1f5ce3eb0 ffffffc1f5ce3eb0 3ec0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3ee0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3f00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3f20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3f40: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3f60: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3f80: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3fa0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 3fc0: 0000000000000000 0000000000000005 0000000000000000 0000000000000000 3fe0: 0000000000000000 0000000000000000 0000000275ce3ff0 0000000275ce3ff8 Call trace: Exception stack(0xffffffc1f5ce3b30 to 0xffffffc1f5ce3c60) 3b20: dead000000000060 0000008000000000 3b40: ffffffc1f5ce3d00 ffffff80084cb694 0000000000000008 0000000000000e88 3b60: ffffffc1f5ce3bb0 ffffff80080dac68 ffffffc1f5ce3b90 ffffff8008826fe4 3b80: 00000000000001c0 00000000000001c0 ffffffc1f5ce3bb0 ffffff800848dfcc 3ba0: 0000000000020000 ffffff8008b15ae4 ffffffc1f5ce3c00 ffffff800808f000 3bc0: 0000000000000010 ffffff80088377f0 ffffffc1f5ccd590 0000000000000140 3be0: dead000000000100 dead000000000200 0000000000000001 0000000000000000 3c00: 0000000000000000 ffffff8008ce2020 ffffffc1f5cc3338 ffffffc1f5ce0000 3c20: 0000000000000840 0000000000000001 0000000000000000 ffffffffffffffff 3c40: ffffffffffffffff ffffffffffffffff 0000000000000001 0000000000000007 [] pl330_irq_handler+0x27c/0x390 [] irq_forced_thread_fn+0x38/0x88 [] irq_thread+0x140/0x200 [] kthread+0xd8/0xf0 [] ret_from_fork+0x10/0x40 Code: f2a00838 f9405763 aa1c03e1 aa1503e0 (f9000443) ---[ end trace f50005726d31199c ]--- Kernel panic - not syncing: Fatal exception in interrupt SMP: stopping secondary CPUs SMP: failed to stop secondary CPUs 0-1 Kernel Offset: disabled Memory Limit: none ---[ end Kernel panic - not syncing: Fatal exception in interrupt I tried to fix this via re-starting with the list-head after dropping the lock then re-takeing it. That was verified that it worked well. I read the code in the function that the lock is dropped for, but it was not clear to me what was the original reason to do it. I even removed the unlock/lock sequence as a test, and it did also solve the panic. As to these two solutions above, we prefer the first one. But it seems stupid to delete elements off the list by the means of re-starting with the list-head. For the second solution, deleting the unlock/lock sequence, though there is a potential dead lock condition, it will never happen as it is in the context of irqs disabled. I'm not sure which of the two solutions is appropriate, maybe neither. I consulted the technical reference of PL330 and the code of its driver, but still not understanding the design completely. So, what do you think, any ideas ? Thanks your for time. Reviewed-by: Zhang Xiao Signed-off-by: Qi Hou --- drivers/dma/pl330.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c index d7327fd..de1fd59 100644 --- a/drivers/dma/pl330.c +++ b/drivers/dma/pl330.c @@ -1510,7 +1510,7 @@ static void pl330_dotask(unsigned long data) /* Returns 1 if state was updated, 0 otherwise */ static int pl330_update(struct pl330_dmac *pl330) { - struct dma_pl330_desc *descdone, *tmp; + struct dma_pl330_desc *descdone; unsigned long flags; void __iomem *regs; u32 val; @@ -1588,7 +1588,9 @@ static int pl330_update(struct pl330_dmac *pl330) } /* Now that we are in no hurry, do the callbacks */ - list_for_each_entry_safe(descdone, tmp, &pl330->req_done, rqd) { + while (!list_empty(&pl330->req_done)) { + descdone = list_first_entry(&pl330->req_done, + struct dma_pl330_desc, rqd); list_del(&descdone->rqd); spin_unlock_irqrestore(&pl330->lock, flags); dma_pl330_rqcb(descdone, PL330_ERR_NONE); -- 2.7.4