On Thu, 28 Apr 2011 14:35:32 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Wed, 27 Apr 2011, John Stultz wrote:
>
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 7b80d54..d37414e 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -124,11 +124,15 @@ ext4_file_write(struct kiocb *iocb, const struct iovec *iov,
> > static unsigned long unaligned_warn_time;
> >
> > /* Warn about this once per day */
> > - if (printk_timed_ratelimit(&unaligned_warn_time, 60*60*24*HZ))
> > + if (printk_timed_ratelimit(&unaligned_warn_time, 60*60*24*HZ)) {
> > + char comm[TASK_COMM_LEN];
> > +
> > + get_task_comm(comm, current);
> > ext4_msg(inode->i_sb, KERN_WARNING,
> > "Unaligned AIO/DIO on inode %ld by %s; "
> > "performance will be poor.",
> > - inode->i_ino, current->comm);
> > + inode->i_ino, comm);
> > + }
> > mutex_lock(ext4_aio_mutex(inode));
> > ext4_aiodio_wait(inode);
> > }
>
> Thanks very much for looking into concurrent readers of current->comm,
> John!
>
> This patch in the series demonstrates one of the problems with using
> get_task_comm(), however: we must allocate a 16-byte buffer on the stack
> and that could become risky if we don't know its current depth. We may be
> particularly deep in the stack and then cause an overflow because of the
> 16 bytes.
>
> I'm wondering if it would be better for ->comm to be protected by a
> spinlock (or rwlock) other than ->alloc_lock and then just require readers
> to take the lock prior to dereferencing it? That's what is done in the
> oom killer with task_lock(). Perhaps you could introduce new
> task_comm_lock() and task_comm_unlock() to prevent the extra stack usage
> in over 300 locations within the kernel?
16 bytes isn't all that much. It's just two longs worth.
I'm suspecting that approximately 100% of the get_task_comm() callsites
are using it for a printk, so how about we add a %p thingy for it then
zap lots of code?
I read the changelogs and can't work out why a seqlock was added. What
was wrong with the task_lock()?
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