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Date: Wed, 01 Feb 2012 17:31:39 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: Dmitry Antipov <dmitry.antipov@...aro.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Module/kthread/printk question/problem
Le mercredi 01 février 2012 à 20:09 +0400, Dmitry Antipov a écrit :
> I'm writing a kernel module which creates a substantial amount of
> kernel threads. After dropping some real stuff, the module is:
>
> #include <linux/kernel.h>
> #include <linux/sched.h>
> #include <linux/kthread.h>
> #include <linux/module.h>
> #include <linux/slab.h>
>
> MODULE_LICENSE("GPL");
>
> static int nrthreads = 128;
> module_param(nrthreads, int, 0644);
>
> static int loopcount = 1024;
> module_param(loopcount, int, 0644);
>
> static int usehrtime = 0;
> module_param(usehrtime, int, 0644);
>
> static int slack = 50000;
> module_param(slack, int, 0644);
>
> static int msecs = 1;
> module_param(msecs, int, 0644);
>
> static DECLARE_COMPLETION(done);
> static struct task_struct **threads;
> static atomic_t nrunning;
>
> static int test(void *unused)
> {
> int i;
> ktime_t expires = ktime_set(0, msecs * NSEC_PER_MSEC);
>
> for (i = 0; !kthread_should_stop() && i < loopcount; i++) {
> if (usehrtime) {
> set_current_state(TASK_UNINTERRUPTIBLE);
> schedule_hrtimeout_range(&expires, slack, HRTIMER_MODE_REL);
> }
> else
> schedule_timeout_uninterruptible(msecs_to_jiffies(msecs));
> }
>
> if (atomic_dec_and_test(&nrunning)) {
> printk("last thread done\n");
> complete(&done);
Race is here :
Here you allow testmod_exit() to continue, while _this_ thread has not
yet exited.
> }
And if this thread is preempted a litle bit, its code already was freed.
-> crash.
> return 0;
> }
>
> static int __init testmod_init(void)
> {
> int i;
>
> printk("test begin\n");
>
> atomic_set(&nrunning, nrthreads);
>
> threads = kmalloc(nrthreads * sizeof(struct task_struct *), GFP_KERNEL);
> if (!threads)
> return -ENOMEM;
>
> for (i = 0; i < nrthreads; i++) {
> threads[i] = kthread_run(test, NULL, "test/%d", i);
> if (IS_ERR(threads[i])) {
> int j, err = PTR_ERR(threads[i]);
>
> for (j = 0; j < i; j++)
> kthread_stop(threads[j]);
> kfree(threads);
> return err;
> }
> }
> return 0;
> }
>
> static void __exit testmod_exit(void)
> {
> wait_for_completion(&done);
> kfree(threads);
> }
>
> module_init(testmod_init);
> module_exit(testmod_exit);
>
> Usually it works as expected, at least from 8 to 128 threads.
> But when I'm trying to run it a loop like:
>
> while true; do insmod testmod.ko && rmmod testmod.ko; sleep 1; done
>
> it's also possible to catch a very rare crash (ARM example):
>
> Unable to handle kernel paging request at virtual address 7f1200c4
> pgd = 80004000
> [7f1200c4] *pgd=bdc28811, *pte=00000000, *ppte=00000000
> Internal error: Oops: 80000007 [#1] PREEMPT SMP
> Modules linked in: [last unloaded: testmod]
> CPU: 1 Tainted: G O (3.3.0-rc2 #3)
> PC is at 0x7f1200c4
> LR is at __schedule+0x684/0x6e4
> pc : [<7f1200c4>] lr : [<802c053c>] psr: 600f0113
> sp : bf115f88 ip : 00000000 fp : 00000000
> r10: 00000000 r9 : 00000000 r8 : 7f120394
> r7 : 00000002 r6 : 00000400 r5 : 7f120204 r4 : bf114000
> r3 : 00000000 r2 : bf115ec0 r1 : bf9220c0 r0 : 00000001
> Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment kernel
> Control: 10c5387d Table: bfbcc04a DAC: 00000015
> Process test/126 (pid: 10918, stack limit = 0xbf1142f8)
> Stack: (0xbf115f88 to 0xbf116000)
> 5f80: 000f4240 00000000 bf213e4c 00000000 7f120000 00000013
> 5fa0: 00000000 80049228 00000000 00000000 00000000 00000000 00000000 00000000
> 5fc0: dead4ead ffffffff ffffffff 8048b2b8 00000000 00000000 8036a3f9 bf115fdc
> 5fe0: bf115fdc 271aee1c bf213e4c 8004919c 8000eabc 8000eabc bfefc811 bfefcc11
> Code: bad PC value
>
> Note the bad PC, and stack is just a nonsense. I suspect that the kernel calls
> testmod_exit() and frees module memory _before_ all test/X threads are really
> dead - i.e. the module memory is freed when at least one of the test/X threads
> is somewhere in do_exit() or nearby. Is that possible? If yes, what's the better
> way to ensure that all test/X threads are really gone at some point of
> testmod_exit()?
>
> An interesting thing is that I can't reproduce this fault with both printk()s
> commented out. No ideas why.
>
> Dmitry
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