diff options
author | Jason Baron <jbaron@redhat.com> | 2005-09-09 20:01:57 (GMT) |
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committer | Linus Torvalds <torvalds@g5.osdl.org> | 2005-09-09 20:57:31 (GMT) |
commit | ff55fe2075e3901db4dbdc00e0b44a71bef97afd (patch) | |
tree | c6dfc8ba5d04fda4c5cb15cdebd5425ab674f8df /drivers/char/pty.c | |
parent | 69ac59647e66c1b53fb98fe8b6d0f2099cffad60 (diff) | |
download | linux-fsl-qoriq-ff55fe2075e3901db4dbdc00e0b44a71bef97afd.tar.xz |
[PATCH] pty_chars_in_buffer oops fix
The idea of this patch is to lock both sides of a ptmx/pty pair during line
discipline changing. This is needed to ensure that say a poll on one side of
the pty doesn't occur while the line discipline is actively being changed.
This resulted in an oops reported on lkml, see:
http://marc.theaimsgroup.com/?l=linux-kernel&m=111342171410005&w=2
A 'hacky' approach was previously implmemented which served to eliminate the
poll vs. line discipline changing race. However, this patch takes a more
general approach to the issue. The patch only adds locking on a less often
used path, the line-discipline changing path, as opposed to locking the
ptmx/pty pair on read/write/poll paths.
The patch below, takes both ldisc locks in either order b/c the locks are both
taken under the same spinlock(). I thought about locking the ptmx/pty
separately, such as master always first but that introduces a 3 way deadlock.
For example, process 1 does a blocking read on the slave side. Then, process
2 does an ldisc change on the slave side, which acquires the master ldisc lock
but not the slave's. Finally, process 3 does a write which blocks on the
process 2's ldisc reference.
This patch does introduce some changes in semantics. For example, a line
discipline change on side 'a' of a ptmx/pty pair, will now wait for a
read/write to complete on the other side, or side 'b'. The current behavior
is to simply wait for any read/writes on only side 'a', not both sides 'a' and
'b'. I think this behavior makes sense, but I wanted to point it out.
I've tested the patch with a bunch of read/write/poll while changing the line
discipline out from underneath.
This patch obviates the need for the above "hide the problem" patch.
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'drivers/char/pty.c')
-rw-r--r-- | drivers/char/pty.c | 5 |
1 files changed, 2 insertions, 3 deletions
diff --git a/drivers/char/pty.c b/drivers/char/pty.c index da32889..49f3997 100644 --- a/drivers/char/pty.c +++ b/drivers/char/pty.c @@ -149,15 +149,14 @@ static int pty_write_room(struct tty_struct *tty) static int pty_chars_in_buffer(struct tty_struct *tty) { struct tty_struct *to = tty->link; - ssize_t (*chars_in_buffer)(struct tty_struct *); int count; /* We should get the line discipline lock for "tty->link" */ - if (!to || !(chars_in_buffer = to->ldisc.chars_in_buffer)) + if (!to || !to->ldisc.chars_in_buffer) return 0; /* The ldisc must report 0 if no characters available to be read */ - count = chars_in_buffer(to); + count = to->ldisc.chars_in_buffer(to); if (tty->driver->subtype == PTY_TYPE_SLAVE) return count; |