Deadlock in /proc/xen/xenbus watch+read on 3.17+ (maybe earlier)

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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Hi,

I've hit some deadlock in kernel xenstore client exposed via
/proc/xen/xenbus. Steps to reproduce are simple:
int main() {
	struct xs_handle *xs;
	xs = xs_open(0);
	xs_watch(xs, "domid", "token");
	xs_read(xs, 0, "name", NULL);
	return 0;
}

xs_watch internally creates new thread, which uses read to wait for the
watch. And in the same time, the program tries to read some value,
but actually it hangs at sending the command (before even sending a path to be
read). Strace gives this (simplified for readability):
[pid  2494] write(3, "\4\0\0\0\0\0\0\0\0\0\0\0\f\0\0\0", 160 = 16
[pid  2494] write(3, "domid\0", 6)      = 6
[pid  2494] write(3, "token\0", 6)      = 6
[pid  2495] read(3,  <unfinished ...>
[pid  2494] futex(0x71c0d4, FUTEX_WAIT_PRIVATE, 1, NULL <unfinished ...>
[pid  2495] <... read resumed>
"\17\0\0\0\377\377\377\377\220~\255\27\f\0\0\0", 16) = 16
[pid  2495] read(3, "domid\0token\0", 12) = 12
[pid  2495] read(3, "\4\0\0\0\0\0\0\0\0\0\0\0\3\0\0\0", 16) = 16
[pid  2495] read(3, "OK\0", 3)          = 3
[pid  2495] futex(0x71c0d4, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0x71c0d0,
{FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1} <unfinished ...>
[pid  2494] <... futex resumed> )       = 0
[pid  2495] <... futex resumed> )       = 1
[pid  2494] futex(0x71c0a8, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid  2495] futex(0x71c0a8, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
[pid  2494] <... futex resumed> )       = -1 EAGAIN (Resource
temporarily unavailable)
[pid  2495] <... futex resumed> )       = 0
[pid  2494] futex(0x71c0a8, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
[pid  2495] read(3,  <unfinished ...>
[pid  2494] <... futex resumed> )       = 0
[pid  2494] rt_sigaction(SIGPIPE, {SIG_DFL, [], SA_RESTORER,
0x7fc78c1488f0}, NULL, 8) = 0
[pid  2494] rt_sigaction(SIGPIPE, {SIG_IGN, [], SA_RESTORER,
0x7fc78c1488f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc78c1488f0}, 8) = 0
[pid  2494] write(3, "\2\0\0\0\0\0\0\0\0\0\0\0\5\0\0\0", 16

And thats all - 2494 is waiting on write, 2495 is waiting on read.

On 3.12.x it is working. On 3.17.0 and 3.18.7 it is broken. I haven't
checked versions in the middle.

Any ideas?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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