Messages in this thread Patch in this message |  | | Subject | Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() | From | Daniel Bristot de Oliveira <> | Date | Thu, 15 Jul 2021 10:22:46 +0200 |
| |
On 7/14/21 12:35 PM, Paolo Bonzini wrote: > On 14/07/21 11:23, Jason Wang wrote: >>> This was added in 2020, so it's unlikely to be the direct cause of the >>> change. What is a known-good version for the host? >>> >>> Since it is not KVM stuff, I'm CCing Michael and Jason. >> >> I think this can be probably fixed here: >> >> https://lore.kernel.org/lkml/20210618084412.18257-1-zhe.he@windriver.com/ > > That seems wrong; in particular it wouldn't protect against AB/BA deadlocks. > In fact, the bug is with the locking; the code assumes that > spin_lock_irqsave/spin_unlock_irqrestore is non-preemptable and therefore > increments and decrements the percpu variable inside the critical section. > > This obviously does not fly with PREEMPT_RT; the right fix should be > using a local_lock. Something like this (untested!!):
the lock needs to be per-pcu... but so far, so good. I will continue using the system in the next days to see if it blows on another way.
The patch looks like this now:
------------------------- 8< --------------------- Subject: [PATCH] eventfd: protect eventfd_wake_count with a local_lock
eventfd_signal assumes that spin_lock_irqsave/spin_unlock_irqrestore is non-preemptable and therefore increments and decrements the percpu variable inside the critical section.
This obviously does not fly with PREEMPT_RT. If eventfd_signal is preempted and an unrelated thread calls eventfd_signal, the result is a spurious WARN. To avoid this, protect the percpu variable with a local_lock.
Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth") Cc: stable@vger.kernel.org Cc: He Zhe <zhe.he@windriver.com> Cc: Jens Axboe <axboe@kernel.dk> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Co-developed-by: Daniel Bristot de Oliveira <bristot@redhat.com> Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com> --- fs/eventfd.c | 27 ++++++++++++++++++++++----- include/linux/eventfd.h | 7 +------ 2 files changed, 23 insertions(+), 11 deletions(-)
diff --git a/fs/eventfd.c b/fs/eventfd.c index e265b6dd4f34..9754fcd38690 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -12,6 +12,7 @@ #include <linux/fs.h> #include <linux/sched/signal.h> #include <linux/kernel.h> +#include <linux/local_lock.h> #include <linux/slab.h> #include <linux/list.h> #include <linux/spinlock.h> @@ -25,8 +26,6 @@ #include <linux/idr.h> #include <linux/uio.h>
-DEFINE_PER_CPU(int, eventfd_wake_count); - static DEFINE_IDA(eventfd_ida);
struct eventfd_ctx { @@ -45,6 +44,20 @@ struct eventfd_ctx { int id; };
+struct event_fd_recursion { + local_lock_t lock; + int count; +}; + +static DEFINE_PER_CPU(struct event_fd_recursion, event_fd_recursion) = { + .lock = INIT_LOCAL_LOCK(lock), +}; + +bool eventfd_signal_count(void) +{ + return this_cpu_read(event_fd_recursion.count); +} + /** * eventfd_signal - Adds @n to the eventfd counter. * @ctx: [in] Pointer to the eventfd context. @@ -71,18 +84,22 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) * it returns true, the eventfd_signal() call should be deferred to a * safe context. */ - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) + local_lock(&event_fd_recursion.lock); + if (WARN_ON_ONCE(this_cpu_read(event_fd_recursion.count))) { + local_unlock(&event_fd_recursion.lock); return 0; + }
spin_lock_irqsave(&ctx->wqh.lock, flags); - this_cpu_inc(eventfd_wake_count); + this_cpu_inc(event_fd_recursion.count); 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); + this_cpu_dec(event_fd_recursion.count); spin_unlock_irqrestore(&ctx->wqh.lock, flags); + local_unlock(&event_fd_recursion.lock);
return n; } diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index fa0a524baed0..ca89d6c409c1 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -43,12 +43,7 @@ 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) -{ - return this_cpu_read(eventfd_wake_count); -} +bool eventfd_signal_count(void);
#else /* CONFIG_EVENTFD */
--
|  |