zohar at linux
Jul 2, 2012, 4:49 AM
Post #19 of 23
(160 views)
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On Mon, 2012-07-02 at 06:11 +0100, Al Viro wrote:
> On Mon, Jul 02, 2012 at 04:43:10AM +0100, Al Viro wrote:
> > On Sun, Jul 01, 2012 at 09:46:31PM -0400, Mimi Zohar wrote:
> > > On Sun, 2012-07-01 at 21:57 +0100, Al Viro wrote:
> > > > On Sun, Jul 01, 2012 at 03:50:02PM -0400, Mimi Zohar wrote:
> > > > > Replacing it with a call to __fput(), the system boots.
> > > >
> > > > "it" being just the part under that if (unlikely(...)))? Very interesting... If so, we
> > > > have some kernel thread ending up with delayed __fput() which somehow makes dracut (assuimg
> > > > you are using fedora initramfs to go with fedora config) unhappy. With your own patch,
> > > > doing async __fput() in a lot of cases when this one doesn't delay past the return to
> > > > userland managing to survive the boot... I wonder which files end up triggering that fun
> > > > and which kernel thread is responsible... Could you slap a printk() in there, showing
> > > > file->f_dentry->d_inode->i_mode (octal) and at least file->f_dentry->d_name.name?
> > > > Along with the current->comm[], all under that inner if (). And see which ones end up
> > > > going that way by the time execve() of /sbin/init fails.
> > >
> > > pid=1 uid=0 d_name=init comm=swapper/0 dev="rootfs" mode=100775
> > > pid=1 uid=0 d_name=bash comm=swapper/0 dev="rootfs" mode=100755
> >
> > OK... Here's what I suspect is going on:
> > * populating initramfs writes binaries there. We open files (for write) from
> > the kernel thread (there's nothing other than kernel threads at that point), write to
> > them, then close(). Final fput() gets delayed.
> > * Then we proceed to execve(). Which means mapping the binary with MAP_DENYWRITE.
> > Which fails, since there's a struct file still opened for write on that sucker.
> >
> > Your patch did not delay those fput() - they were done without ->mmap_sem held. So
> > it survived. Booting without initramfs always survives; booting with initramfs may
> > or may not survive, depending on the timings - if that scheduled work manages to
> > run by the time we do those execve(), we win. Note that async_synchronize_full()
> > done in init_post() might easily affect that, depending on config.
> >
> > As a quick test, could you try slapping a delay somewhere around the beginning
> > of init_post() and see if it rescues the system?
>
> Ho-hum... How about this (modulo missing documentation of the whole sad mess):
Sorry, neither adding the delay or this patch helped.
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 470da0b..00fd849 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -284,6 +284,11 @@ static void ____fput(struct callback_head *work)
> __fput(container_of(work, struct file, f_u.fu_rcuhead));
> }
>
> +void flush_delayed_fput(void)
> +{
> + delayed_fput(NULL);
> +}
> +
> static DECLARE_WORK(delayed_fput_work, delayed_fput);
>
> void fput(struct file *file)
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 58bf158..d9a4f5a 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -39,4 +39,6 @@ extern void put_unused_fd(unsigned int fd);
>
> extern void fd_install(unsigned int fd, struct file *file);
>
> +extern void flush_delayed_fput(void);
> +
> #endif /* __LINUX_FILE_H */
> diff --git a/init/main.c b/init/main.c
> index b5cc0a7..3f151f6 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -68,6 +68,7 @@
> #include <linux/shmem_fs.h>
> #include <linux/slab.h>
> #include <linux/perf_event.h>
> +#include <linux/file.h>
>
> #include <asm/io.h>
> #include <asm/bugs.h>
> @@ -804,8 +805,8 @@ static noinline int init_post(void)
> system_state = SYSTEM_RUNNING;
> numa_default_policy();
>
> -
> current->signal->flags |= SIGNAL_UNKILLABLE;
> + flush_delayed_fput();
>
> if (ramdisk_execute_command) {
> run_init_process(ramdisk_execute_command);
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