kernel-ark/arch/um/os-Linux/signal.c
Jeff Dike 53b173327d [PATCH] uml: fix I/O hang
Fix a UML hang in which everything would just stop until some I/O happened
- a ping, someone whacking the keyboard - at which point everything would
start up again as though nothing had happened.

The cause was gcc reordering some code which absolutely needed to be
executed in the order in the source.  When unblock_signals switches signals
from off to on, it needs to see if any interrupts had happened in the
critical section.  The interrupt handlers check signals_enabled - if it is
zero, then the handler adds a bit to the "pending" bitmask and returns.
unblock_signals checks this mask to see if any signals need to be
delivered.

The crucial part is this:
	signals_enabled = 1;
	save_pending = pending;
	if(save_pending == 0)
		return;
	pending = 0;

In order to avoid an interrupt arriving between reading pending and setting
it to zero, in which case, the record of the interrupt would be erased,
signals are enabled.

What happened was that gcc reordered this so that 'save_pending = pending'
came before 'signals_enabled = 1', creating a one-instruction window within
which an interrupt could arrive, set its bit in pending, and have it be
immediately erased.

When the I/O workload is purely disk-based, the loss of a block device
interrupt stops the entire I/O system because the next block request will
wait for the current one to finish.  Thus the system hangs until something
else causes some I/O to arrive, such as a network packet or console input.

The fix to this particular problem is a memory barrier between enabling
signals and reading the pending signal mask.  An xchg would also probably
work.

Looking over this code for similar problems led me to do a few more
things:

- make signals_enabled and pending volatile so that they don't get cached
  in registers

- add an mb() to the return paths of block_signals and unblock_signals so
  that the modification of signals_enabled doesn't get shuffled into the
  caller in the event that these are inlined in the future.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-11-03 12:27:58 -08:00

251 lines
5.5 KiB
C

/*
* Copyright (C) 2004 PathScale, Inc
* Licensed under the GPL
*/
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <stdarg.h>
#include <string.h>
#include <sys/mman.h>
#include "user_util.h"
#include "user.h"
#include "signal_kern.h"
#include "sysdep/sigcontext.h"
#include "sysdep/barrier.h"
#include "sigcontext.h"
#include "mode.h"
#include "os.h"
/* These are the asynchronous signals. SIGVTALRM and SIGARLM are handled
* together under SIGVTALRM_BIT. SIGPROF is excluded because we want to
* be able to profile all of UML, not just the non-critical sections. If
* profiling is not thread-safe, then that is not my problem. We can disable
* profiling when SMP is enabled in that case.
*/
#define SIGIO_BIT 0
#define SIGIO_MASK (1 << SIGIO_BIT)
#define SIGVTALRM_BIT 1
#define SIGVTALRM_MASK (1 << SIGVTALRM_BIT)
#define SIGALRM_BIT 2
#define SIGALRM_MASK (1 << SIGALRM_BIT)
/* These are used by both the signal handlers and
* block/unblock_signals. I don't want modifications cached in a
* register - they must go straight to memory.
*/
static volatile int signals_enabled = 1;
static volatile int pending = 0;
void sig_handler(int sig, struct sigcontext *sc)
{
int enabled;
enabled = signals_enabled;
if(!enabled && (sig == SIGIO)){
pending |= SIGIO_MASK;
return;
}
block_signals();
CHOOSE_MODE_PROC(sig_handler_common_tt, sig_handler_common_skas,
sig, sc);
set_signals(enabled);
}
static void real_alarm_handler(int sig, struct sigcontext *sc)
{
if(sig == SIGALRM)
switch_timers(0);
CHOOSE_MODE_PROC(sig_handler_common_tt, sig_handler_common_skas,
sig, sc);
if(sig == SIGALRM)
switch_timers(1);
}
void alarm_handler(int sig, struct sigcontext *sc)
{
int enabled;
enabled = signals_enabled;
if(!signals_enabled){
if(sig == SIGVTALRM)
pending |= SIGVTALRM_MASK;
else pending |= SIGALRM_MASK;
return;
}
block_signals();
real_alarm_handler(sig, sc);
set_signals(enabled);
}
void set_sigstack(void *sig_stack, int size)
{
stack_t stack = ((stack_t) { .ss_flags = 0,
.ss_sp = (__ptr_t) sig_stack,
.ss_size = size - sizeof(void *) });
if(sigaltstack(&stack, NULL) != 0)
panic("enabling signal stack failed, errno = %d\n", errno);
}
void remove_sigstack(void)
{
stack_t stack = ((stack_t) { .ss_flags = SS_DISABLE,
.ss_sp = NULL,
.ss_size = 0 });
if(sigaltstack(&stack, NULL) != 0)
panic("disabling signal stack failed, errno = %d\n", errno);
}
void (*handlers[_NSIG])(int sig, struct sigcontext *sc);
extern void hard_handler(int sig);
void set_handler(int sig, void (*handler)(int), int flags, ...)
{
struct sigaction action;
va_list ap;
sigset_t sig_mask;
int mask;
handlers[sig] = (void (*)(int, struct sigcontext *)) handler;
action.sa_handler = hard_handler;
sigemptyset(&action.sa_mask);
va_start(ap, flags);
while((mask = va_arg(ap, int)) != -1)
sigaddset(&action.sa_mask, mask);
va_end(ap);
action.sa_flags = flags;
action.sa_restorer = NULL;
if(sigaction(sig, &action, NULL) < 0)
panic("sigaction failed - errno = %d\n", errno);
sigemptyset(&sig_mask);
sigaddset(&sig_mask, sig);
if(sigprocmask(SIG_UNBLOCK, &sig_mask, NULL) < 0)
panic("sigprocmask failed - errno = %d\n", errno);
}
int change_sig(int signal, int on)
{
sigset_t sigset, old;
sigemptyset(&sigset);
sigaddset(&sigset, signal);
sigprocmask(on ? SIG_UNBLOCK : SIG_BLOCK, &sigset, &old);
return(!sigismember(&old, signal));
}
void block_signals(void)
{
signals_enabled = 0;
/* This must return with signals disabled, so this barrier
* ensures that writes are flushed out before the return.
* This might matter if gcc figures out how to inline this and
* decides to shuffle this code into the caller.
*/
mb();
}
void unblock_signals(void)
{
int save_pending;
if(signals_enabled == 1)
return;
/* We loop because the IRQ handler returns with interrupts off. So,
* interrupts may have arrived and we need to re-enable them and
* recheck pending.
*/
while(1){
/* Save and reset save_pending after enabling signals. This
* way, pending won't be changed while we're reading it.
*/
signals_enabled = 1;
/* Setting signals_enabled and reading pending must
* happen in this order.
*/
mb();
save_pending = pending;
if(save_pending == 0){
/* This must return with signals enabled, so
* this barrier ensures that writes are
* flushed out before the return. This might
* matter if gcc figures out how to inline
* this (unlikely, given its size) and decides
* to shuffle this code into the caller.
*/
mb();
return;
}
pending = 0;
/* We have pending interrupts, so disable signals, as the
* handlers expect them off when they are called. They will
* be enabled again above.
*/
signals_enabled = 0;
/* Deal with SIGIO first because the alarm handler might
* schedule, leaving the pending SIGIO stranded until we come
* back here.
*/
if(save_pending & SIGIO_MASK)
CHOOSE_MODE_PROC(sig_handler_common_tt,
sig_handler_common_skas, SIGIO, NULL);
if(save_pending & SIGALRM_MASK)
real_alarm_handler(SIGALRM, NULL);
if(save_pending & SIGVTALRM_MASK)
real_alarm_handler(SIGVTALRM, NULL);
}
}
int get_signals(void)
{
return signals_enabled;
}
int set_signals(int enable)
{
int ret;
if(signals_enabled == enable)
return enable;
ret = signals_enabled;
if(enable)
unblock_signals();
else block_signals();
return ret;
}
void os_usr1_signal(int on)
{
change_sig(SIGUSR1, on);
}