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Message-Id: <20211217103209.1122679-3-bigeasy@linutronix.de>
Date: Fri, 17 Dec 2021 11:32:09 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: fcarli@...il.com
Cc: linux-kernel@...r.kernel.org,
"Luis Claudio R . Goncalves" <lgoncalv@...hat.com>,
stable-rt@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Jason Wang <jasowang@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/2] eventfd: Make signal recursion protection a task bit
From: Thomas Gleixner <tglx@...utronix.de>
Upstream commit b542e383d8c005f06a131e2b40d5889b812f19c6
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: Daniel Bristot de Oliveira <bristot@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Daniel Bristot de Oliveira <bristot@...hat.com>
Acked-by: Jason Wang <jasowang@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
fs/aio.c | 2 +-
fs/eventfd.c | 12 +++++-------
include/linux/eventfd.h | 11 +++++------
include/linux/sched.h | 4 ++++
4 files changed, 15 insertions(+), 14 deletions(-)
diff --git a/fs/aio.c b/fs/aio.c
index 76ce0cc3ee4ec..51b08ab01dffc 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -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);
diff --git a/fs/eventfd.c b/fs/eventfd.c
index df466ef81dddf..9035ca60bfcf3 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -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;
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index dc4fd8a6644dd..836b4c021a0a4 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -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
@@ -42,11 +43,9 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);
int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
__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 */
@@ -77,9 +76,9 @@ 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;
}
#endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 409a24036952c..29e6ff1af1df9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -852,6 +852,10 @@ struct task_struct {
/* Stalled due to lack of memory */
unsigned in_memstall:1;
#endif
+#ifdef CONFIG_EVENTFD
+ /* Recursion prevention for eventfd_signal() */
+ unsigned in_eventfd_signal:1;
+#endif
unsigned long atomic_flags; /* Flags requiring atomic access. */
--
2.34.1
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