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: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx

diff --git a/fs/aio.c b/fs/aio.c
index 76ce0cc..51b08ab 100644 (file)
--- 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 e265b6d..3627dd7 100644 (file)
--- 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 fa0a524..305d5f1 100644 (file)
--- 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
@@ -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)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3bb9fec..6421a9a 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -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. */