On 05/15/2013 11:47 AM, Joerg Roedel wrote:
On Wed, May 15, 2013 at 05:03:17PM +0200, Jiri Slaby wrote:
On 05/15/2013 12:56 PM, Joerg Roedel wrote:
Are you fixing any bug here? output_lock does not protect
tty->ops->write on the other places, not tty->ops->write.
Yes, I am trying to fix a BUG_ON that triggered in
drivers/tty/hvc/hvc_xen.c in function __write_console(). This function
was called from the place I am patching in this fix. My current
explanation for that BUG_ON is a race condition that comes
from concurrent calls to that function.
That is also the only explanation that makes sense because the
__write_console() function itself makes sure that the condition can not
hit.
In the comment for the n_tty_write function there is this remark:
* Locking: output_lock to protect column state and space left
* (note that the process_output*() functions take this
* lock themselves)
So the space left is managed in the ->write callback and needs
protection.
nack.
"space left" is not honored when OPOST is clear, so it is not protected
in this case. IOW, tty->ops->write_room() is not called, so by-definition
there is "space left".
Are you certain your stack trace takes you through this particular
invocation of tty->ops->write()? Could it be that the compiler has
inlined process_output_block() into n_tty_write() and that's what your
seeing?
Can you attach the BUG report?
Are you certain OPOST is cleared? (output of stty -a -F </dev/xxxx>)
Is CONFIG_CONSOLE_POLL=y?
Is this happening during boot or much later?
The process_output*() functions all (unless I am missing something)
take the output_lock before calling the tty->ops->write (directly or
indirectly).
The place I patched here is the only place in n_tty_write where the
->write call-back is invoked directly, and it happens without taking the
lock. I think this is a problem.
But not the only path to __write_console().
For example, what serializes hvc_console_print() with hvc_write()
for the same console index?
Regards,
Peter Hurley
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