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17 May 2024, 11:00:25 UTC
Revision 09bf4f5b6e6013f0ad6b090d4a8deebd4e56d878 authored by Al Viro on 15 June 2017, 04:17:30 UTC, committed by Al Viro on 15 June 2017, 04:41:18 UTC 
ufs: avoid grabbing ->truncate_mutex if possible
 
tail unpacking is done in a wrong place; the deadlocks galore
is best dealt with by doing that in ->write_iter() (and switching
to iomap, while we are at it), but that's rather painful to
backport.  The trouble comes from grabbing pages that cover
the beginning of tail from inside of ufs_new_fragments(); ongoing
pageout of any of those is going to deadlock on ->truncate_mutex
with process that got around to extending the tail holding that
and waiting for page to get unlocked, while ->writepage() on
that page is waiting on ->truncate_mutex.

The thing is, we don't need ->truncate_mutex when the fragment
we are trying to map is within the tail - the damn thing is
allocated (tail can't contain holes).

Let's do a plain lookup and if the fragment is present, we can
just pretend that we'd won the race in almost all cases.  The
only exception is a fragment between the end of tail and the
end of block containing tail.

Protect ->i_lastfrag with ->meta_lock - read_seqlock_excl() is
sufficient.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
1 parent 267309f
Raw File
Tip revision: 09bf4f5b6e6013f0ad6b090d4a8deebd4e56d878 authored by Al Viro on 15 June 2017, 04:17:30 UTC
ufs: avoid grabbing ->truncate_mutex if possible
Tip revision: 09bf4f5
nmi_backtrace.c 
/*
 *  NMI backtrace support
 *
 * Gratuitously copied from arch/x86/kernel/apic/hw_nmi.c by Russell King,
 * with the following header:
 *
 *  HW NMI watchdog support
 *
 *  started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
 *
 *  Arch specific calls to support NMI watchdog
 *
 *  Bits copied from original nmi.c file
 */
#include <linux/cpumask.h>
#include <linux/delay.h>
#include <linux/kprobes.h>
#include <linux/nmi.h>
#include <linux/cpu.h>
#include <linux/sched/debug.h>

#ifdef arch_trigger_cpumask_backtrace
/* For reliability, we're prepared to waste bits here. */
static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;

/* "in progress" flag of arch_trigger_cpumask_backtrace */
static unsigned long backtrace_flag;

/*
 * When raise() is called it will be passed a pointer to the
 * backtrace_mask. Architectures that call nmi_cpu_backtrace()
 * directly from their raise() functions may rely on the mask
 * they are passed being updated as a side effect of this call.
 */
void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
				   bool exclude_self,
				   void (*raise)(cpumask_t *mask))
{
	int i, this_cpu = get_cpu();

	if (test_and_set_bit(0, &backtrace_flag)) {
		/*
		 * If there is already a trigger_all_cpu_backtrace() in progress
		 * (backtrace_flag == 1), don't output double cpu dump infos.
		 */
		put_cpu();
		return;
	}

	cpumask_copy(to_cpumask(backtrace_mask), mask);
	if (exclude_self)
		cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));

	/*
	 * Don't try to send an NMI to this cpu; it may work on some
	 * architectures, but on others it may not, and we'll get
	 * information at least as useful just by doing a dump_stack() here.
	 * Note that nmi_cpu_backtrace(NULL) will clear the cpu bit.
	 */
	if (cpumask_test_cpu(this_cpu, to_cpumask(backtrace_mask)))
		nmi_cpu_backtrace(NULL);

	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
		pr_info("Sending NMI from CPU %d to CPUs %*pbl:\n",
			this_cpu, nr_cpumask_bits, to_cpumask(backtrace_mask));
		raise(to_cpumask(backtrace_mask));
	}

	/* Wait for up to 10 seconds for all CPUs to do the backtrace */
	for (i = 0; i < 10 * 1000; i++) {
		if (cpumask_empty(to_cpumask(backtrace_mask)))
			break;
		mdelay(1);
		touch_softlockup_watchdog();
	}

	/*
	 * Force flush any remote buffers that might be stuck in IRQ context
	 * and therefore could not run their irq_work.
	 */
	printk_safe_flush();

	clear_bit_unlock(0, &backtrace_flag);
	put_cpu();
}

bool nmi_cpu_backtrace(struct pt_regs *regs)
{
	int cpu = smp_processor_id();

	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
		if (regs && cpu_in_idle(instruction_pointer(regs))) {
			pr_warn("NMI backtrace for cpu %d skipped: idling at pc %#lx\n",
				cpu, instruction_pointer(regs));
		} else {
			pr_warn("NMI backtrace for cpu %d\n", cpu);
			if (regs)
				show_regs(regs);
			else
				dump_stack();
		}
		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
		return true;
	}

	return false;
}
NOKPROBE_SYMBOL(nmi_cpu_backtrace);
#endif
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