Messages in this thread | | | Date | Thu, 19 Sep 2002 03:59:40 -0700 | From | William Lee Irwin III <> | Subject | Re: [patch] generic-pidhash-2.5.36-D4, BK-curr |
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On Thu, Sep 19, 2002 at 11:25:11AM +0200, Ingo Molnar wrote: > i'll add some debugging code to the old code to print out cases when > p->tty == tty but p->session != tty->session and start up X, that should > prove or disprove this theory, right? > (i cant remember any other place where the code transformation was not > identity, but will double-check this again.) > William, you did the original transformation, was this optimization done > intentionally? > Ingo
Sorry, I was debugging some boot-time issues I ran into.
I did this intentionally. Basically, sys_setsid() does the right thing, but tty_ioctl() does not. There is already some inconsistency about how task->tty is locked, and I'd not yet come to a conclusion.
On second thought, I really hope I'm imagining things here:
For instance, tty_open() does the following while not holding the tasklist_lock for writing:
if (IS_TTY_DEV(device)) { if (!current->tty) return -ENXIO; device = current->tty->device;
So tiocsctty() appears to have the following race against tty_open():
A B ----------------------------------------- tty->session > 0 if (!current->tty) return -ENXIO; (test fails, don't return)
iterate tasklist, delay... setting p->tty = NULL current->tty is set to NULL
(this looks like an attempt to obtain a unique reference by blowing away everyone else's)
task_lock(current) device = current->tty->device current->tty = tty OOPS task_unlock(task)
tiocsctty() holds the BKL (from sys_ioctl() itself) and the tasklist_lock for reading, but tty_open() seems to hold only some vfs references. release_dev() does the same style of iteration, but doesn't even hold the BKL, only the tasklist_lock for reading. And is very ominously called by tty_open() itself.
Also, most other accesses to tty->session and/or tty->pgrp seem to be protected by the BKL.
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