[Bug 287501] Observing witness warning w.r.t rcu_read_lock() API

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=287501

            Bug ID: 287501
           Summary: Observing witness warning w.r.t rcu_read_lock() API
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: sreekanth.reddy@broadcom.com

Created attachment 261234
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=261234&action=edit
patch which fixes the witness warning related rcu_read_lock()

Observing below witness calltrace while running KTLS enabled traffic.

uma_zalloc_debug: zone "lkpicurr" with the following non-sleepable locks held:
exclusive rw tcpinp (tcpinp) r = 0 (0xfffff809e6146020) locked @
/usr/srreddy/freebsd-src/sys/kern/uipc_ktls.c:195
stack backtrace:
#0 0xffffffff80bee42c at witness_debugger+0x6c
#1 0xffffffff80bef640 at witness_warn+0x430
#2 0xffffffff80f34044 at uma_zalloc_debug+0x34
#3 0xffffffff80f33b97 at uma_zalloc_arg+0x27
#4 0xffffffff80e2a029 at linux_alloc_current+0x69
#5 0xffffffff80e370a3 at linux_rcu_read_lock+0x183
#6 0xffffffff82f4ad55 at bnxt_tls_snd_tag_free+0xe5
#7 0xffffffff80b4bec0 at m_snd_tag_destroy+0x10
#8 0xffffffff80c19e77 at ktls_destroy+0x2e7
#9 0xffffffff80c1d18f at ktls_work_thread+0xf0f


According to rcu_read_lock() definition this API should be a NON-Preemptable,
but I see that uma_zalloc_arg is called without M_NOWAIT flag set and it leads
to above witness warning.

I have made below kernel changes which fixed this witness warnings. So please
review it.

diff --git a/sys/compat/linuxkpi/common/include/linux/sched.h
b/sys/compat/linuxkpi/common/include/linux/sched.h
index 3ad2f8e4ce8b..30a61a9e806c 100644
--- a/sys/compat/linuxkpi/common/include/linux/sched.h
+++ b/sys/compat/linuxkpi/common/include/linux/sched.h
@@ -98,6 +98,12 @@ struct task_struct {
        ((struct task_struct *)__td->td_lkpi_task); \
 })

+#define current_nowait  ({ \
+        struct thread *__td = curthread; \
+        linux_set_current_flags(__td, M_NOWAIT); \
+        ((struct task_struct *)__td->td_lkpi_task); \
+})
+
 #define        task_pid_group_leader(task) (task)->task_thread->td_proc->p_pid
 #define        task_pid(task)          ((task)->pid)
 #define        task_pid_nr(task)       ((task)->pid)
diff --git a/sys/compat/linuxkpi/common/src/linux_rcu.c
b/sys/compat/linuxkpi/common/src/linux_rcu.c
index c0b864d269b3..cd6db8e47b91 100644
--- a/sys/compat/linuxkpi/common/src/linux_rcu.c
+++ b/sys/compat/linuxkpi/common/src/linux_rcu.c
@@ -188,7 +188,7 @@ linux_rcu_read_lock(unsigned type)
        if (RCU_SKIP())
                return;

-       ts = current;
+       ts = current_nowait;

        /* assert valid refcount */
        MPASS(ts->rcu_recurse[type] != INT_MAX);
@@ -226,11 +226,11 @@ linux_rcu_read_unlock(unsigned type)
        if (RCU_SKIP())
                return;

-       ts = current;
+       ts = current_nowait;

        /* assert valid refcount */
        MPASS(ts->rcu_recurse[type] > 0);
-
+
        if (--(ts->rcu_recurse[type]) != 0)
                return;

Thanks,
Sreekanth

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