[PATCH 4.4 117/137] dump_stack: avoid potential deadlocks

Greg Kroah-Hartman
2016-02-24T04:18:43Z

From: Eric Dumazet <edumazet@google.com>

commit d7ce36924344ace0dbdc855b1206cacc46b36d45 upstream.

Some servers experienced fatal deadlocks because of a combination of
bugs, leading to multiple cpus calling dump_stack().

The checksumming bug was fixed in commit 34ae6a1aa054 ("ipv6: update
skb->csum when CE mark is propagated").

The second problem is a faulty locking in dump_stack()

CPU1 runs in process context and calls dump_stack(), grabs dump_lock.

CPU2 receives a TCP packet under softirq, grabs socket spinlock, and
call dump_stack() from netdev_rx_csum_fault().

dump_stack() spins on atomic_cmpxchg(&dump_lock, -1, 2), since
dump_lock is owned by CPU1

While dumping its stack, CPU1 is interrupted by a softirq, and happens
to process a packet for the TCP socket locked by CPU2.

CPU1 spins forever in spin_lock() : deadlock

Stack trace on CPU1 looked like :

NMI backtrace for cpu 1
RIP: _raw_spin_lock+0x25/0x30
...
Call Trace:
<IRQ>
tcp_v6_rcv+0x243/0x620
ip6_input_finish+0x11f/0x330
ip6_input+0x38/0x40
ip6_rcv_finish+0x3c/0x90
ipv6_rcv+0x2a9/0x500
process_backlog+0x461/0xaa0
net_rx_action+0x147/0x430
__do_softirq+0x167/0x2d0
call_softirq+0x1c/0x30
do_softirq+0x3f/0x80
irq_exit+0x6e/0xc0
smp_call_function_single_interrupt+0x35/0x40
call_function_single_interrupt+0x6a/0x70
<EOI>
printk+0x4d/0x4f
printk_address+0x31/0x33
print_trace_address+0x33/0x3c
print_context_stack+0x7f/0x119
dump_trace+0x26b/0x28e
show_trace_log_lvl+0x4f/0x5c
show_stack_log_lvl+0x104/0x113
show_stack+0x42/0x44
dump_stack+0x46/0x58
netdev_rx_csum_fault+0x38/0x3c
__skb_checksum_complete_head+0x6e/0x80
__skb_checksum_complete+0x11/0x20
tcp_rcv_established+0x2bd5/0x2fd0
tcp_v6_do_rcv+0x13c/0x620
sk_backlog_rcv+0x15/0x30
release_sock+0xd2/0x150
tcp_recvmsg+0x1c1/0xfc0
inet_recvmsg+0x7d/0x90
sock_recvmsg+0xaf/0xe0
___sys_recvmsg+0x111/0x3b0
SyS_recvmsg+0x5c/0xb0
system_call_fastpath+0x16/0x1b

Fixes: b58d977432c8 ("dump_stack: serialize the output from dump_stack()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
lib/dump_stack.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -25,6 +25,7 @@ static atomic_t dump_lock = ATOMIC_INIT(

asmlinkage __visible void dump_stack(void)
{
+ unsigned long flags;
int was_locked;
int old;
int cpu;
@@ -33,9 +34,8 @@ asmlinkage __visible void dump_stack(voi
* Permit this cpu to perform nested stack dumps while serialising
* against other CPUs
*/
- preempt_disable();
-
retry:
+ local_irq_save(flags);
cpu = smp_processor_id();
old = atomic_cmpxchg(&dump_lock, -1, cpu);
if (old == -1) {
@@ -43,6 +43,7 @@ retry:
} else if (old == cpu) {
was_locked = 1;
} else {
+ local_irq_restore(flags);
cpu_relax();
goto retry;
}
@@ -52,7 +53,7 @@ retry:
if (!was_locked)
atomic_set(&dump_lock, -1);

- preempt_enable();
+ local_irq_restore(flags);
}
#else
asmlinkage __visible void dump_stack(void)