Commit b542e383 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

eventfd: Make signal recursion protection a task bit 



The recursion protection for eventfd_signal() is based on a per CPU
variable and relies on the !RT semantics of spin_lock_irqsave() for
protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
disables preemption nor interrupts which allows the spin lock held section
to be preempted. If the preempting task invokes eventfd_signal() as well,
then the recursion warning triggers.

Paolo suggested to protect the per CPU variable with a local lock, but
that's heavyweight and actually not necessary. The goal of this protection
is to prevent the task stack from overflowing, which can be achieved with a
per task recursion protection as well.

Replace the per CPU variable with a per task bit similar to other recursion
protection bits like task_struct::in_page_owner. This works on both !RT and
RT kernels and removes as a side effect the extra per CPU storage.

No functional change for !RT kernels.

Reported-by: default avatarDaniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarDaniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: default avatarJason Wang <jasowang@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx
parent 366e7ad6

fs/aio.c

+1 −1
Original line number Diff line number Diff line
@@ -1695,7 +1695,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync, 
		list_del(&iocb->ki_list);

		iocb->ki_res.res = mangle_poll(mask);

		req->done = true;

		if (iocb->ki_eventfd && eventfd_signal_count()) {

		if (iocb->ki_eventfd && eventfd_signal_allowed()) {

			iocb = NULL;

			INIT_WORK(&req->work, aio_poll_put_work);

			schedule_work(&req->work);

fs/eventfd.c

+5 −7
Original line number Diff line number Diff line
@@ -25,8 +25,6 @@ 
#include <linux/idr.h>

#include <linux/uio.h>



DEFINE_PER_CPU(int, eventfd_wake_count);



static DEFINE_IDA(eventfd_ida);



struct eventfd_ctx {
@@ -67,21 +65,21 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) 
	 * Deadlock or stack overflow issues can happen if we recurse here

	 * through waitqueue wakeup handlers. If the caller users potentially

	 * nested waitqueues with custom wakeup handlers, then it should

	 * check eventfd_signal_count() before calling this function. If

	 * it returns true, the eventfd_signal() call should be deferred to a

	 * check eventfd_signal_allowed() before calling this function. If

	 * it returns false, the eventfd_signal() call should be deferred to a

	 * safe context.

	 */

	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))

	if (WARN_ON_ONCE(current->in_eventfd_signal))

		return 0;



	spin_lock_irqsave(&ctx->wqh.lock, flags);

	this_cpu_inc(eventfd_wake_count);

	current->in_eventfd_signal = 1;

	if (ULLONG_MAX - ctx->count < n)

		n = ULLONG_MAX - ctx->count;

	ctx->count += n;

	if (waitqueue_active(&ctx->wqh))

		wake_up_locked_poll(&ctx->wqh, EPOLLIN);

	this_cpu_dec(eventfd_wake_count);

	current->in_eventfd_signal = 0;

	spin_unlock_irqrestore(&ctx->wqh.lock, flags);



	return n;

include/linux/eventfd.h

+5 −6
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ 
#include <linux/err.h>

#include <linux/percpu-defs.h>

#include <linux/percpu.h>

#include <linux/sched.h>



/*

 * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
@@ -43,11 +44,9 @@ int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *w 
				  __u64 *cnt);

void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt);



DECLARE_PER_CPU(int, eventfd_wake_count);



static inline bool eventfd_signal_count(void)

static inline bool eventfd_signal_allowed(void)

{

	return this_cpu_read(eventfd_wake_count);

	return !current->in_eventfd_signal;

}



#else /* CONFIG_EVENTFD */
@@ -78,9 +77,9 @@ static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, 
	return -ENOSYS;

}



static inline bool eventfd_signal_count(void)

static inline bool eventfd_signal_allowed(void)

{

	return false;

	return true;

}



static inline void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)

include/linux/sched.h

+4 −0
Original line number Diff line number Diff line
@@ -864,6 +864,10 @@ struct task_struct { 
	/* Used by page_owner=on to detect recursion in page tracking. */

	unsigned			in_page_owner:1;

#endif

#ifdef CONFIG_EVENTFD

	/* Recursion prevention for eventfd_signal() */

	unsigned			in_eventfd_signal:1;

#endif



	unsigned long			atomic_flags; /* Flags requiring atomic access. */