From: Paul Ruizendaal <pnr@planet.nl>
To: tuhs@minnie.tuhs.org
Subject: Re: [TUHS] V6 networking & alarm syscall
Date: Sat, 12 Jan 2019 12:13:24 +0100 [thread overview]
Message-ID: <D10210D4-B796-44C5-8862-866B5FFD6CBC@planet.nl> (raw)
In-Reply-To: <mailman.1.1547258402.6716.tuhs@minnie.tuhs.org>
> / uses the system sleep call rather than the standard C library
> / sleep (sleep (III)) because there is a critical race in the
> / C library implementation which could result in the process
> / pausing forever. This is a horrible bug in the UNIX process
> / control mechanism.
>
> Quoted without comment from me!
Intriguing comment. I think your v6+ system probably has a lot of
PWB stuff in there. The libc source for sleep() in stock V6 is:
.globl _sleep
sleep = 35.
_sleep:
mov r5,-(sp)
mov sp,r5
mov 4(r5),r0
sys sleep
mov (sp)+,r5
rts pc
The PWB version uses something alarm/pause based, but apparently
gets it wrong:
.globl _sleep
alarm = 27.
pause = 29.
rti = 2
_sleep:
mov r5,-(sp)
mov sp,r5
sys signal; 14.; 1f
mov 4(r5),r0
sys alarm
sys pause
clr r0
sys alarm
mov (sp)+,r5
rts pc
1:
rti
I think the race occurs when an interrupt arrives between the sys alarm
and the sys pause lines, and the handler calls sleep again.
sleep() in the V7 libc is a much more elaborate C routine.
When I first read the race condition comment, I thought the issue would
be like that of write:
_write:
mov r5,-(sp)
mov sp,r5
mov 4(r5),r0
mov 6(r5),0f
mov 8(r5),0f+2
sys 0; 9f
bec 1f
jmp cerror
1:
mov (sp)+,r5
rts pc
.data
9:
sys write; 0:..; ..
This pattern appears in several V6 sys call routines, and would
not be safe when used in a context with signal based multi-
threading.
next parent reply other threads:[~2019-01-12 11:14 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.1.1547258402.6716.tuhs@minnie.tuhs.org>
2019-01-12 11:13 ` Paul Ruizendaal [this message]
2019-01-13 10:52 Paul Ruizendaal
2019-01-13 15:39 ` Warner Losh
-- strict thread matches above, loose matches on Subject: below --
2019-01-12 4:14 Noel Chiappa
2019-01-12 1:24 Noel Chiappa
2019-01-12 1:58 ` Dave Horsfall
2019-01-12 2:33 ` Warner Losh
2019-01-11 23:27 Paul Ruizendaal
2019-01-10 23:12 Paul Ruizendaal
2019-01-11 0:17 ` Clem cole
2019-01-11 15:33 ` ron
2019-01-11 18:45 ` Clem Cole
2019-01-11 19:06 ` David
2019-01-11 22:08 ` reed
2019-01-11 22:18 ` Larry McVoy
2019-01-12 12:04 ` reed
2019-01-12 17:20 ` Clem Cole
2019-01-12 18:16 ` Eric Allman
2019-01-13 1:16 ` Madeline Autumn-Rose
2019-01-11 22:47 ` Clem Cole
2019-01-11 22:55 ` Eric Allman
2019-01-11 23:32 ` Warner Losh
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